Preheat oven to 475º F.
In a small bowl, combine all ingredients for the spice blend.
In a large bowl, toss the strips of onion and peppers with 1 tablespoon oil, making sure the vegetables are all evenly coated with oil. Sprinkle 4 teaspoons of the spice blend over the vegetables (reserving about 2 3/4 teaspoons of the spice blend for the shrimp). Toss to coat until the spices are evenly distributed.
Divide the vegetables onto two large, parchment-lined baking sheets, spreading the veggies out so they're not piled on top of each other. Roast vegetables for 7-8 minutes.
While the vegetables start roasting, in the large bowl you used for the vegetables, toss the shrimp with remaining 1 tablespoon olive oil to coat. Sprinkle the shrimp with the remaining spice blend, and toss again to evenly distribute the spices.
After the vegetables have roasted for 7-8 minutes, remove baking sheets from the oven. Stir the vegetables and push them toward the edges of the sheet pans, to make space in the center of each sheet pan to add shrimp. (Make sure the vegetables still have a little room to spread out instead of being piled up). Add a few shrimp to the center of each baking sheet, spreading them out a little.
Return baking sheets to the oven and continue roasting for about 5 minutes more, until the shrimp are opaque and just cooked through (the exact amount of time can vary slightly depending on the size of your shrimp – see note). Check carefully to be sure the shrimp don't overcook.
Serve immediately with tortillas and desired toppings. (You may want to transfer the shrimp and veggies from the baking sheets to a large serving platter, so the shrimp don't continue to cook on the hot baking sheets.)
Peppers and onions: If you can't find really large peppers, you can use two smaller red and two smaller green ones. The goal is to end up with approximately 2 1/2 cups of each type of sliced pepper, and 4 cups of sliced onion. Also, I've made this recipe with only green pepper when that's all I had – feel free to mix up pepper colors based on what's available. In addition, I've tested these fajitas with a yellow Spanish onion instead of the red onion, and that's a great substitution, although not quite as colorful.
Shrimp sizes: You'll often find shrimp sized by numbers, such as "21-25." Those numbers give you a rough idea of how large the shrimp are, by indicating approximately how many shrimp are in a pound. I like to use medium-large shrimp that are in the 21-25 or 26-30 size range (meaning I'll have about 21-30 shrimp in a pound). If you are using larger or smaller shrimp, you'll need to adjust the roasting time slightly. If your shrimp are fairly small, for example, you may want to increase the initial roasting time for the veggies, and then decrease the second roasting time once you've added the shrimp, since smaller shrimp won't need to cook as long. Just be sure to watch the shrimp carefully and check on them early – so they don't over-roast and become tough. The shrimp are done when they have just turned opaque, but are still tender.
Make-ahead steps: To make this meal even faster on busy nights, you can measure and combine the spices in advance. You can also slice your vegetables and store them in the refrigerator a day or two ahead.