Slice the steak thinly across the grain. Stir together the lime juice, garlic, chili powder, cumin, paprika, salt, pepper, and 1 tablespoon of the oil, then toss with the steak and let it stand for 15 to 30 minutes; refrigerate if marinating longer.

Blanch the trimmed green beans in boiling salted water for 3 to 4 minutes, until bright green and barely tender. Drain and rinse briefly under cold water so they stop cooking while retaining their fresh texture.

Heat a large cast-iron skillet or heavy frying pan over high heat. Add the remaining 1 tablespoon oil once the pan is hot.

Cook the steak in a single layer, working in batches if necessary, for about 2 to 3 minutes per side or until browned and cooked to your desired doneness. Transfer the cooked steak to a plate.

Add the sliced onion and bell pepper to the hot skillet. Cook for 3 to 5 minutes, stirring occasionally, until the vegetables have lightly charred edges while remaining slightly crisp.

Add the blanched green beans and cook for 2 to 3 minutes more. Return the steak to the pan and toss everything together for about 1 minute, just until heated through.

Serve the fajita mixture immediately in the 8 warmed flour tortillas. Add sour cream, shredded Monterey Jack or cheddar, salsa or pico de gallo, avocado, cilantro, and lime wedges as desired.