"From-scratch" culinary purists have laid on enough guilt to push convenient old standbys from the cupboard into the closet. It's a bold cook who's willing to admit he or she likes to mix a packet of onion soup into a tub of sour cream for party dip. But even my friend Joan Nightingale (who loves to cook) pulls an envelope out of her closet regularly. "My family thinks the pot roast I make with oxtail soup mix and a can of beer is the best ever," she declares. "I think it's the easiest." She has some points. You be the judge.