Mist the bottom of a pot with nonstick cooking spray. Add the onions and garlic. Add the tomatoes, red pepper flakes, rind, stock, and collard greens. Stir to combine.
Bring to a boil then reduce the heat to simmer. Cook until tender, about 20 minutes. Taste and adjust for seasoning with salt and pepper. Keeps up to 3 days in an airtight container.