Preheat and ready the oven: Preheat the oven to 425°F (220°C). A high oven temperature encourages the sweet potatoes to caramelize on the outside while staying tender inside — that contrast is what makes great fries.

Prepare the sweet potato fries: Wash and peel the sweet potatoes (peeling is optional; skin adds texture if you prefer). Cut into even 1/2-inch (about 1 cm) sticks so they cook at the same rate. Uniform size prevents some pieces from burning while others remain undercooked.

Soak and dry for crispness: Place the cut fries in cold water for 10 minutes (this removes some surface starch and helps prevent them from sticking together). Drain and pat completely dry — moisture is the enemy of crisp fries, so be thorough here.

Toss with oil and season: In a bowl, toss the dried fries with 1 tbsp olive oil, a generous pinch of salt and a few grinds of black pepper. The oil promotes browning; salt early on seasons the potatoes through and pepper adds background heat.

Bake the fries: Spread fries in a single layer on a rimmed baking sheet lined with parchment or a lightly oiled baking rack. Crowding the pan creates steam, which will soften rather than crisp the fries. Roast for 20–25 minutes, turning once halfway, until edges are golden and caramelized. If you want extra blistering, broil 1–2 minutes at the end — watch closely to avoid burning.

Slice the onion: While the fries start, halve and thinly slice the large onion into half-moons. Thin slices will collapse and caramelize more evenly and quickly.

Caramelize the onions: Heat a large skillet over medium-low heat and add 1 tbsp butter plus a drizzle (about 1 tsp) of olive oil — the oil raises the butter’s smoke point while butter adds the rich flavor needed for great caramelization. Add the sliced onions and a pinch of salt; cook slowly, stirring every few minutes so the sugars brown evenly and don’t burn. This should take 20–25 minutes. If the pan becomes dry, add a tablespoon of water to loosen browned bits — that fond adds flavor.

Finish the onions with garlic and balsamic: When the onions are soft and golden-brown, push them to the side, add the minced garlic to the pan for 30 seconds to bloom the garlic flavor (browning garlic too long makes it bitter). Stir the garlic into the onions, add 1 tsp balsamic vinegar to deglaze and concentrate the sweetness, then taste and adjust salt and pepper. The balsamic brightens the sweetness and gives the onions a glossy finish.

Bring the patties to room temperature: While the onions caramelize, remove the bison patties from the refrigerator for 10–15 minutes so they warm slightly. This helps them cook evenly — cold meat tightens and can overcook on the outside before the center reaches the right temperature.

Season the patties: Pat the patties dry with paper towel and season both sides simply with salt and freshly ground black pepper. Bison has a clean, slightly sweet beefy flavor that needs only light seasoning.

Grill or pan-sear the bison: Preheat a grill or a heavy cast-iron skillet over medium-high heat until very hot. Bison is lean, so a hot sear quickly develops a flavorful crust without overcooking. Add the patties and cook 3–4 minutes per side for medium-rare to medium depending on thickness — aim for 130–135°F (55–57°C) for medium-rare, 140–145°F (60–63°C) for medium. Because it's lean, avoid cooking much beyond 145°F or it will dry out.

Rest the patties: Transfer cooked patties to a plate and tent loosely with foil for 5 minutes. Resting lets the muscle fibers relax and juices redistribute, keeping the meat juicy when you cut into it.

Assemble: Arrange 3 lettuce leaves per serving on plates as a bed (these act as a light 'bun'), place a grilled bison patty on top, add 2–3 spoonfuls of caramelized onions and 2–3 tomato slices. Serve the sweet potato fries alongside and season fries with a final pinch of salt right out of the oven to heighten flavor.

Serve and enjoy: Offer optional condiments like Dijon mustard, a smear of whole-grain mustard, or a dollop of Greek yogurt mixed with lemon and chopped chives. These provide acid or creaminess to balance the richness of the meat and the sweetness of the onions and fries.