In a large skillet over medium heat, add the olive oil. Once the oil is hot, add the cubed chicken breast. Season with kosher salt and freshly ground black pepper. Cook for about 5-7 minutes, stirring occasionally, until the chicken is cooked through and golden brown.
Add the diced onion, minced garlic, and sliced red bell pepper to the skillet with the chicken. Sauté for an additional 3-4 minutes, until the vegetables are softened and fragrant.
In a mixing bowl, whisk together the eggs and half-and-half until well combined. Season with a pinch of salt and pepper.
Push the chicken and vegetable mixture to one side of the skillet. Add the butter to the empty side and allow it to melt. Pour the egg mixture into the skillet and let it sit for a few moments before gently stirring. Scramble the eggs until they are cooked but still soft and creamy.
Warm the flour tortillas in a separate skillet or microwave. Once warm, place a generous scoop of the chicken and egg mixture in the center of each tortilla. Top with shredded cheddar and Monterey Jack cheese, diced tomatoes, diced avocado, and fresh spinach.
Fold the sides of the tortilla over the filling, then roll from the bottom up to enclose the filling completely. Serve the burritos warm with your favorite hot sauce drizzled on top.