THREE-DAY WEEKENDS probably should bear responsibility for the invention of brunch. What better way to celebrate an extra no-deadline morning than sleeping late, and what better way to follow sleeping late than catching up on the meal you missed.

If you like to combine invention with a strong dose of tradition, try sweet potato waffles for this more-or-less breakfast. They've got a strong dose of nutrition, too.

Make sure to get to the market ahead of time for a few ingredients to check through the express lane, and look to your shelves for the salt, flour and shortening you'll need, plus butter if you demand it on your waffles. And cook the sweet potatoes the night before, or better yet serve these nutritious vegetables for dinner and make enough to leave for brunch.

EXPRESS LANE LIST: Baking powder, cinnamon, eggs, milk, sweet potatoes, walnuts, syrup. SWEET POTATO WAFFLES (4 to 6 servings) 2 cups flour 3 teaspoons baking powder 1 teaspoon salt 1/4 teaspoon cinnamon 3 eggs, separated 1 1/2 cups milk 3/4 cup mashed cooked sweet potatoes 1/4 cup shortening, melted, or oil 1/3 cup chopped walnuts Syrup and butter for serving, if desired

Sift flour, baking powder, salt and cinnamon together. Beat egg yolks well; add milk and sweet potatoes and beat until blended. Add to dry ingredients and mix well. Add shortening or oil and mix well. Beat egg whites until stiff but not dry, then fold into sweet potato mixture. Pour on preheated waffle iron and sprinkle with nuts. Bake 5 to 7 minutes or until done. Serve with syrup and butter if desired.