In this recipe, you could substitute cottage cheese for ricotta. But do note, when you whip ricotta cheese, the texture changes completely. It becomes light and fluffy. If you are used to using it in savory recipes, the addition of lemon and sugar, makes this a delicious side for the brioche french toast. If neither of these substitutes work, you could serve some lemon and sugar sweetened Greek yogurt on the side or for a decadent brunch item or dessert, a lemon flavored whipped cream would be great.