Combine all the spices in a small bowl. Toss half of this blend with the chicken pieces until evenly coated. Reserve the remaining mix for the vegetables.
Heat one tablespoon of olive oil in a large 12-inch nonstick skillet over medium-high heat until shimmering. Arrange the chicken in a single layer and sear for 2-3 minutes per side until a golden crust forms and the chicken is cooked through. Remove the chicken, cover lightly, and keep warm.
Reduce the heat to medium and add the remaining olive oil to the same skillet. Toss in the onions, bell peppers, and mushrooms, then sprinkle over the reserved seasoning. Sauté the vegetables for about 4-5 minutes, stirring occasionally. If the pan feels dry, add 2-3 tablespoons of water or broth to help deglaze and bring out extra flavor.
Assemble your fiesta bowl by layering the seared chicken and vegetables over a bed of hot rice (if using). Top with generous amounts of shredded cheese, sour cream, guacamole or chopped avocados, your favorite salsa, chopped cilantro, roasted corn kernels, and a handful of crushed tortilla chips. Squeeze a fresh lime wedge on top for an extra burst of brightness.