Preheat the oven to 400 degrees.
Remove the stems from the mushroom and chop them up into very small pieces.
Heat the oil in a skillet over medium heat. Add the garlic and cook until softened, 1 minute. Add the mushrooms stems and salt and pepper to taste. Cook, stirring until the mixture is softened, and well combined, about 10 minutes. Add stock or water if the mixture dries out.
Remove from the heat and mix in the breadcrumbs. Add a little stock or water if the mixture seems too try.
Add a little stock or water to the bottom of a shallow baking pan or casserole dish. Stuff the mushroom caps with the stuffing and arrange them in the pan. Bake until the mushrooms are tender and the stuffing is hot, about 20 minutes.