1Heat the oven to 450°F with a rack in the upper third and let a large rimmed sheet pan preheat in there. A hot pan gives the steak and peppers a head start on browning.
2Stir the chili powder, cumin, smoked paprika, garlic powder, oregano, kosher salt, and a few grinds of black pepper together in a small bowl. This is your fajita seasoning.
3In a large bowl, toss the steak strips with 1 tbsp vegetable oil, the garlic, and 2 tbsp of the seasoning. In a second large bowl, toss the peppers and onion with 1 tbsp vegetable oil and 1 tbsp of the seasoning. Keep the two proteins in separate bowls.
4In a third bowl, toss the shrimp with the remaining 1 tbsp vegetable oil and the remaining seasoning, plus the juice of one lime. Set it in the fridge. It stays cold and waits because the shrimp cook in only a few minutes and would overcook if they went in now.
5Carefully pull the hot pan from the oven. Spread the peppers and onion over one half of the pan and the steak strips over the other half in a single layer, with a little space between pieces. Roast for 10 minutes. The peppers need this long to soften and char at the edges, and the thick steak strips need it to reach medium-rare to medium.
6Pull the pan out and push the steak and vegetables toward the center to open up room. Scatter the shrimp in a single layer along the cleared areas. This is the staggered step: the shrimp go in now because they need only 4 to 5 minutes, and adding them at the start would leave them tough and dry by the time the steak is done.
7Roast another 4 to 5 minutes, until the shrimp are pink and curled into a loose C and the steak is browned at the edges. For a little extra char, switch to broil for the last 60 seconds and watch closely.
8Squeeze the juice from the remaining lime over everything on the pan and scatter with cilantro. Let the steak rest on the pan for 2 minutes.
9Serve straight from the pan with warm tortillas, sour cream, and lime wedges. Let everyone build their own fajitas with a mix of steak, shrimp, and peppers.