Prepare the marinated flank steak according to its recipe or package directions, then wrap tightly in foil and let rest for 5–10 minutes.
While the steak rests, heat 1 tablespoon olive oil in a medium skillet over medium-high heat.
Add the sliced onion, sliced red pepper, and sliced mushrooms to the hot skillet and cook, stirring frequently, until vegetables are tender and slightly browned, about 5–7 minutes; remove from heat.
Mix 1/2 cup sour cream (or Greek yogurt) with 1 teaspoon Sriracha in a small bowl; adjust Sriracha to taste and set the sauce aside.
Slice the rested flank steak thinly against the grain.
Assemble each lettuce wrap by layering slices of steak and some sautéed vegetables on a large lettuce leaf, and drizzle or dollop with the Sriracha cream; fold and serve immediately.