Step 1 Soak porcini mushrooms in water 30 minutes.
Step 2 Heat butter and oil in large (14-cup capacity) nonaluminum pot or casserole over medium-high heat. When hot, add onions; stir well to coat. Cook, stirring often, until softened, about 10 minutes. Add paprika, salt and pepper to taste. Stir well. Cook 2 minutes.
Step 3Drain porcini mushrooms through strainer lined with triple thickness of paper towels, reserving liquid. Chop on board or in mini-processor. Stem and halve white mushrooms (cut into quarters if large). Cut beef into 1 1/3-inch cubes. Add porcini and white mushrooms and meat to pot. Cook until heated through, stirring often, about 5 minutes. Puree tomatoes with liquid and add to pot with bell pepper, stock and reserved mushroom liquid. Stir well.
Step 4Bring to boil, then reduce heat and simmer, covered, stirring occasionally, about 1 1/2 hours. At this point, if there is too much liquid, simmer, uncovered, to cook down this liquid, stirring often to avoid any scorching, about 30 to 40 minutes more. Add a little beef stock or broth if goulash is too thick. Meat should be very tender. Remove from heat. Taste; adjust seasoning. (To store, cool on counter, then refrigerate at least overnight or up to 3 days. Can also be frozen up to 1 month. To serve, reheat, covered, in 350-degree oven for 1 hour. Stir; taste and adjust seasoning.) Serve hot over hot Buttered Noodles With Garlicky Bread Crumbs.