Cut broccoli stalks into 1/2-inch chunks and put into a food processor. Pulse until stalks resemble rice.
Melt butter in a medium skillet over medium-low heat. Add garlic and stir for about 1 minute. Increase heat to medium and add broccoli rice. Cook, stirring often, until tender, 5 to 7 minutes. Turn off heat, stir in lemon zest and red pepper if using and season with salt and pepper. Serve topped with Parmesan.