Preheat oven to 400°F.
Brush both sides of the salmon fillets with 1 tablespoon of the olive oil and place them on a baking sheet, skin side down (if there is skin).
Combine the brown sugar, chili powder, ground cumin, red pepper flakes, salt, smoked paprika, and oregano. Mix well.
Season the tops of the salmon fillets with half of the spice mixture.
Add the peppers, onion, and garlic to a medium bowl and toss with the remaining tablespoon of olive oil and spice mixture.
Spread the vegetables around the salmon fillets on the baking sheet.
Place the sheet in the preheated oven and bake, tossing halfway through, for 20-25 minutes, or until the salmon flakes away easily with a fork and the vegetables are tender.
Garnish with the cilantro and lime slices, if desired.