Trim and measure: Remove any large visible gristle from the bison patties (if present). Pat them dry with paper towel and season both sides with 3/4 teaspoon kosher salt and 1/4 teaspoon freshly ground black pepper. Patting dry encourages an immediate Maillard sear (browning) when the meat hits a hot pan or grill; seasoning ahead flavors the meat without drawing out too much moisture.

Prep the fries: Preheat the oven to 425°F (220°C). Line a large rimmed baking sheet with foil or parchment and place a second baking sheet or inverted rimmed pan on the lower rack to preheat (this creates a hot surface for crisper fries). Peel (optional) and cut the sweet potatoes into uniform sticks about 1/3"–1/2" (8–12 mm) thick. Uniform pieces cook evenly; thinner fries crisp faster, thicker fries stay tender inside.

Soak and dry the fries: Place the cut sweet potato sticks in a bowl of cold water and let them sit for 10–15 minutes to draw surface starch off (this helps develop a crisper exterior). Drain and pat completely dry with clean kitchen towels or paper towels — moisture on the surface will steam the fries and prevent crisping.

Toss with oil and salt: In a bowl, toss the dried sweet potatoes with 1 tablespoon olive oil and 1/2 teaspoon kosher salt, spreading in a single layer on the preheated baking sheet without crowding. A hot sheet and a single layer give immediate contact heat so the edges crisp rather than steam.

Start the fries in the oven: Roast the fries at 425°F (220°C) for 12 minutes, then flip or shake the pan and roast for another 10–12 minutes until the fries are deep golden at the edges and tender through. If you have a convection function, use it to speed browning. Timing varies with thickness—watch for color more than the clock.

Slice the onion: While the fries roast, cut the onion in half pole-to-pole, peel, then slice into 1/8"–1/4" (3–6 mm) half-moons. Thin, even slices caramelize more quickly and evenly.

Sear the bison to collect drippings: Heat a heavy skillet or cast-iron pan over medium-high until hot. Place the bison patties in the pan and sear 2–3 minutes per side without moving to develop a deep brown crust (the Maillard reaction adds flavor). Bison is leaner than beef, so sear quickly and do not overcook. Aim for 125–130°F (52–54°C) for rare, 135°F (57°C) for medium-rare; carryover heat will raise it a few degrees while resting.

Use the pan drippings to start the onions: After searing the patties, remove them to a plate and tent loosely with foil to rest. Reduce heat to medium-low and add the sliced onions to the same pan; the fond (browned bits) and rendered fat left behind carry intense flavor into the onions. If the pan is dry, add a tablespoon of water to deglaze and scrape up the browned bits.

Caramelize the onions slowly: Cook the onions over medium-low, stirring every 2–3 minutes for about 12–18 minutes until soft and beginning to color. Sprinkle a pinch more salt to help draw moisture out. If they start to stick, add 1–2 tablespoons water to loosen the fond and encourage even caramelization. Lower heat and continue until deep golden-brown (total 20–25 minutes). Slow, gentle cooking converts onion sugars and yields sweet, silky results without burning.

Finish the bison and fries: If the burgers need a touch more cooking to reach your target temperature, return them to the pan off center or into a 375°F (190°C) oven for a minute or two while the onions finish. Remove the fries when they are golden and crisp, seasoning with a final pinch of salt while hot to help it stick.

Rest and assemble: Let burgers rest 4–5 minutes — resting lets juices redistribute so they aren’t lost when you cut into the meat. On each plate, arrange lettuce leaves as a base (these act as a bun), place a bison patty on top, add 2–3 spoonfuls of caramelized onions, a couple of tomato slices, and finish with a sprinkle of flaky salt and freshly ground black pepper. Serve fries on the side.

Serving suggestion and timing: Serve immediately while the fries are hot and the burger is still warm. Offer a sharp mustard, lemony yogurt or a garlic-herb aioli on the side - these acidic/savory contrasts highlight the lean bison meat.