In a small bowl, mix together the chili powder, smoked paprika, paprika, onion powder, garlic powder, oregano, salt & pepper. (Set some of the dry rub seasoning to sprinkle on the vegetables in Step #6.) Place the shrimp on a plate and season both sides with the dry rub. Set the shrimp aside.
Preheat a cast iron griddle (or large pan) over medium heat on your grill or stovetop.
Pour one tablespoon of oil on the griddle and add the onions. Let the onions cook down slowly as the cast iron heats up, stirring occasionally (about 5 minutes). Turn the heat to medium-high, then add the peppers and jalapeños. Cook, stirring occasionally, until vegetables are tender-firm and slightly charred on the edges.
Remove the vegetables to a plate, set aside.
Add a tablespoon of olive oil to the griddle, then add the shrimp, without overcrowding them. Cook shrimp for 1 ½ - 2 minutes, flip and cook for 1 ½- 2 minutes more (until shrimp are cooked through). Remove the shrimp to a separate plate, turn off the grill.
Then add back the peppers, onions & jalapeños back on the griddle and toss in some of the dry rub seasoning. Add the shrimp on top of the vegetables and serve on the sizzling hot griddle.
Serve the warmed flour tortillas and toppings on the side and let everyone assemble their own fajitas.