Heat a large skillet over medium heat and add the olive oil.
Season the cubed chicken with Kosher salt and freshly ground black pepper, then cook in the skillet until golden brown and cooked through, about 6–8 minutes; remove chicken to a plate and set aside.
Add the diced onion, minced garlic, and sliced red bell pepper to the same skillet; season lightly with salt and cook until the vegetables are tender, about 5 minutes.
In a medium bowl, whisk the eggs with the half and half until combined and season with salt and pepper.
Wipe out the skillet, return it to medium heat, and melt the butter. When it begins to foam, reduce heat to medium-low and pour in the egg mixture.
Gently scramble the eggs with a rubber spatula, stirring occasionally, until soft curds form and the eggs are just set; season with salt and pepper to taste.
Warm the tortillas if desired, then assemble each burrito by placing ¼ of the cooked chicken in the center, followed by ¼ of the cooked vegetables, ¼ of the scrambled eggs, about ¼ cup each of cheddar and Monterey Jack cheese, and then top with diced tomato, spinach, and diced avocado.
Fold the two sides of each tortilla over the filling and roll tightly. Serve immediately with hot sauce.
