Spinach-, Tomato-, and Cheese-Stuffed Mushrooms

This filling little snack is gluten-free, keto-friendly, and vegetarian, making it a great choice for get-togethers when you don’t know if any guests are on special diets. You can easily measure a cup of frozen spinach when you buy it chopped and loose in the bag, rather than the type that is frozen into a block. Measure before it thaws because it will shrink significantly. Some stores sell mushroom caps ready for stuffing—with stems removed and uniform in size. If buying a carton of miscel