In a small bowl, whisk together the cumin, smoked paprika, onion powder, chili powder, black pepper, and dried oregano. This step blooms the spices together and ensures they're evenly distributed throughout the marinade rather than clumping when wet ingredients are added. Set aside.
In a large mixing bowl, whisk together the olive oil, lime juice, water, sugar, and kosher salt until the sugar and salt are fully dissolved. This creates a balanced base with acidity, fat, and seasoning. Add the minced garlic and the spice mixture from Step 1, whisking well to combine. I like to let this sit for a minute so the garlic can start to infuse the liquid—it creates a more developed flavor than if you immediately add the chicken.
Add the chicken, bell peppers, and onions to the marinade bowl, stirring well to ensure all pieces are coated evenly. Fold in the fresh cilantro last so it stays vibrant. Cover and refrigerate for at least 30 minutes for a quick marinade, or up to 8 hours for deeper flavor development. I find that even 2-3 hours gives you excellent results without requiring advance planning. The acidic lime juice will gently cook the surface of the chicken while the spices penetrate the meat.