Several hours or the day before serving, prepare polenta triangles. Grease a 9-inch-square baking pan. Finely chop 2 onion. In 2-quart saucepan, saute chopped onions in 1 tsp. Olive oil until golden. Carefully stir in mil, then stir in cornmeal, 3/4 tsp. Salt, and 1/4 tsp. Pepper. Heat polenta mixture to boiling over medium heat, stirring occasionally. Reudce heat to low, cover, and cook without stirring for 5 minutes.

In small bowl, lightly beat egg. Gradually stir a little of the hot polenta into the gg; then stir the egg-and-polenta mixture into the pot of polenta. Cook, stirring constantly, 1 minute. Pour polenta into greased pan. Spread to make surface level. Cool to room temperature on wire rack. Cover and refrigerate until firm-several hours or overhight.

Three hours before serving, prepare beef filling: Heat oven to 350°F. In large skillet, heat remaining oil. Saute beef cubes, half at a time, until well browned on all sides. Remove beef cubes to 1 1/2-quart casserole or souffle dish as browned.

In same skillet, saute remaining 6 onion, the mushrooms, and garlic until golden. Remove to casserole and toss with beef.

Carefully add 1 cup water to skillet; heat to boiling. In cup or small bowl, stir together remaining 1/2 cup water, the flour, mustard, the remaining 3/4 tsp. Salt, and 1/4 tsp. Pepper. Stir into boiling water in skillet to make sauce. Cook, stirring to loosen browned-on bits, until thickened. Stir in olives and capers. Pour sauce over beef mixture in casserole. Cover and bake 1 1/2 hours.

With sharp knife, cut polenta into 4 1/2-inch squares; cut squares diagonally in half to make 8 triangles. Remove lid from casserole; arrange polenta triangles, spoke-fashion, on top of beef mixture. Sprinkle with cheeses.

Bake casserole, uncovered, 30 minutes longer or until beef is tender and polenta triangles are browned on edges. Serve from casserole.