In a large skillet, saute onion in oil and butter for 2 minutes. Add garlic; cook 1 minute longer. Add mushrooms; cook 6-8 minutes longer or until mushrooms are tender and no liquid remains. Stir in salt and pepper.
In a large bowl, combine the eggs, bread crumbs, Parmesan cheese, parsley, thyme and mushroom mixture. Crumble beef and sausage over mixture; mix well.
On a large piece of heavy-duty foil, pat beef mixture into a 15x10-in. rectangle. Layer the prosciutto, Havarti, basil and tomatoes to within 1 in. of edges. Roll up jelly-roll style, starting with a short side and peeling foil away while rolling. Seal seams and ends.
Place seam side down in a greased 13x9-in. baking dish. Bake, uncovered, at 350° for 75-85 minutes or until no pink remains and a thermometer reads 160°. Let stand for 5 minutes. Using two large spatulas, carefully transfer meat loaf to a serving platter.