Mushrooms serve as buns.
In a bowl combine all ingredients through red bell pepper. Shape into 2 patties. In a non-stick frying pan fry patties 3-4 minutes on each side until cooked through (or grill). You can grill the mushroom caps if you want, but it is not necessary. The cap is treated like a bun. Mix the mayonnaise, yogurt, mustard and dill weed together. Dress the portabello with mayonnaise mixture, lettuce, tomato and onion. Serve with pickles and Giardiniera salad. Add a shared apple for a sweet ending.