Cook the brown rice according to the package directions. Transfer to a large bowl and cool completely.
Meanwhile, heat the oil in a large skillet over medium-high heat. Add the onions and cook, stirring frequently, until soft and lightly browned, about 4 minutes. Add the tomato paste, chile powder and cumin and cook, stirring, until fragrant, about 1 minute. Add the black beans, squash, 1 cup water and 1/2 teaspoon salt. Bring to a simmer and cook until the beans are soft and most of the liquid has reduced but the mixture is still slightly saucy, 8 to 10 minutes. Add the spinach, in batches if needed, and fold in until wilted. Allow to cool completely.
Lay the tortillas out on a clean work surface. Put 1/2 cup of the rice in the center of each tortilla, about 1 inch from the edges, and top with about 1/2 cup of the bean filling. Sprinkle each with 2 heaping tablespoons Cheddar, and add a few dashes of hot sauce. Fold the edge nearest to you over the filling, fold in the sides and finish rolling up. Wrap tightly first in plastic wrap and then in foil, and freeze for up to 2 weeks.
To reheat in a microwave: Wrap the frozen burrito in parchment paper and microwave on high until hot in the center, 4 to 5 minutes.
For the crunchy oven method: Preheat the oven to 425 degrees F. Spray each side of the burrito generously with cooking spray and place seam-side down on a baking sheet. Cook until golden and crispy on the outside and hot inside, turning about halfway through, 25 to 30 minutes.
Serve with reduced-fat sour cream, salsa and guacamole if desired.