Known in Italy as sformato (sfor-MAH-toa), this dish can be a savory pie, pudding, custard, or soufflé. Our potato version is fairly dense and soufflélike with a crusty top. Cut it into wedges, and add a ladleful of gravy.
Cook potato in boiling salted water to cover 25 minutes or until tender; drain well. Return potato to pan; mash with a potato masher.
Melt 6 tablespoons butter in a heavy saucepan over medium heat; whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in milk; cook over medium heat, whisking constantly, until sauce is thickened and bubbly. Remove from heat; cool.
Melt 3 tablespoons butter in a large skillet over medium heat; add onion, and sauté 10 minutes or until tender and browned.
Stir together mashed potato, onion, white sauce, beaten egg yolks, and next 4 ingredients. Mash with potato masher until smooth.
Butter a 9" x 3" springform pan. Add 1/2 cup breadcrumbs, tilting to coat bottom and sides of pan. Freeze 1 minute to set crumbs. Spoon potato filling into pan. Top with remaining 1/2 cup breadcrumbs.
Bake, uncovered, at 400° for 50 minutes or until set. Cool 15 minutes before removing sides of pan. Garnish, if desired. Serve warm.
Note: Make fresh breadcrumbs easily by tearing a few pieces from a French baguette. Pulse bread in a food processor or mini chopper until you get coarse crumbs.