Cook the rice: Rinse the rice in a fine-mesh sieve under cold water until the water runs clear. In a medium saucepan, combine the rice with 2 cups of water and a pinch of salt. Bring to a boil, then reduce heat to low, cover, and simmer for 30 minutes, or until the rice is cooked through and fluffy.
Cook the chicken: Choose one of these methods:
Grilling: Preheat a grill pan or outdoor grill to medium-high heat. Season the chicken breasts with salt, pepper, and any other desired spices. Grill for 5-7 minutes per side, or until cooked through.
OR Baking: Preheat oven to 400°F (200°C). Season the chicken breasts with salt, pepper, and spices. Place on a baking sheet lined with parchment paper and bake for 15-20 minutes, or until cooked through.
Assemble the bowls: Divide the cooked rice, black beans, lettuce, and mashed avocado evenly among serving bowls. Top each bowl with a sliced or shredded chicken breast, cheddar cheese, and a dollop of salsa.