Bring the steak to room temperature (20–30 minutes). Pat dry with paper towels and season both sides with 1 tsp kosher salt and 1/2 tsp freshly ground black pepper. Bringing the meat to room temperature ensures even cooking; patting it dry and salting promotes a better sear by removing surface moisture.

Preheat your oven to 425°F (220°C). While the oven heats, char the bell peppers: place whole peppers on a rimmed baking sheet and roast 18–22 minutes, turning occasionally, until skins are blackened and blistered. Roasting concentrates the pepper’s sweetness and makes the skin easy to peel, which keeps the sauce smooth and bright.

When peppers are blistered, transfer to a bowl and cover tightly with plastic wrap or a lid for 10 minutes to steam — this loosens the skins. Meanwhile, thinly slice the onions. The steam step makes peeling effortless and prevents the purée from becoming bitter from charred skin.

While peppers steam, caramelize the onions: heat 1 tbsp olive oil and 1 tbsp butter in a wide skillet over medium-low heat. Add the sliced onions and a pinch (1/4 tsp) of salt. Cook, stirring every 3–4 minutes, for 20–25 minutes until deep golden and jammy. Turning the heat low gives the onions time to break down and develop sweet, complex caramelized sugars rather than burning.

About 8 minutes before the onions are done, heat a large cast-iron or heavy skillet on high until very hot—this is essential for a proper sear. Add 1 tbsp olive oil and place the steak in the pan. Sear without moving for 2–3 minutes to develop a deep brown crust, then flip and sear the other side 2–3 minutes. For a 1.5-lb flank, sear times like this should produce medium-rare to medium depending on thickness (use an instant-read thermometer: 125–130°F for rare, 130–135°F for medium-rare, 140°F for medium). High heat caramelizes the surface proteins and creates flavor via the Maillard reaction.

After searing, transfer the steak to a rimmed baking sheet and in the same skillet add 1/2 cup beef stock and 1 tbsp balsamic vinegar, scraping up any brown bits (fond) for 30 seconds. Pour the liquid into the baking sheet around the steak and place in the preheated oven for 4–8 minutes, depending on desired doneness. Finishing in the oven gives controlled, even cooking without burning the crust.

Remove steak from oven and transfer to a cutting board; tent loosely with foil and rest 8–10 minutes. Resting lets the muscle fibers relax and the juices redistribute so the slices remain juicy rather than leaking onto the cutting board.

While the steak rests and onions finish, peel the peppers: remove charred skin, stem, and seeds (they should pull away easily after steaming). In a blender or food processor, combine the peeled roasted peppers, the smashed garlic, smoked paprika (if using), 1/4 tsp salt, remaining 1/4 tsp black pepper, and a splash (1–2 tbsp) of the onion pan juices or stock. Purée until smooth — you want a silky sauce that complements the steak's richness. Taste and adjust acidity with a little more balsamic if needed.

Finish the pepper sauce in a small saucepan over low heat: add the purée and 1 tbsp butter, warm gently and whisk to create a glossy emulsion. Heating slowly preserves the bright pepper flavor and the butter adds sheen and mouthfeel.

Thinly slice the rested steak across the grain into 1/4-inch slices. Slicing against the grain shortens the muscle fibers and makes each bite tender.

To plate: spoon a pool of roasted bell pepper sauce, fan slices of steak on top, and spoon a nest of caramelized onions alongside. Sprinkle with chopped parsley and a final pinch of flaky sea salt. Serve immediately.