In a small bowl, mix the cumin through the red pepper flake (if using).
Put the shrimp in a large bowl with 2 tbs olive oil and all but 1 tbs of the spice mix. Add the garlic and mix.
Heat the remaining olive oil in a pan (medium heat). Add the onion and cook until it starts to soften and brown (around 5 minutes).
Add the bell peppers and the remaining spice blend. Allow to cook for just another 3-5, until the peppers are warm, but not soggy.
Transfer the pepper and onion blend to a plate.
Return the skillet to medium-high heat and add the shrimp. Cook until the shrimp are cooked through, 3 to 4 minutes.
Add the sautéed vegetables back to the pan, toss to combine and heat through.
Drizzle with lime juice and serve with cilantro and lime wedges.