Place a baking sheet on bottom rack of oven. Heat oven to 400°.
Peel rutabaga. Cut a thin slice from 1 side, allowing it to stand upright. Cut into 1/4-inch-thick slices.
Stir together breadcrumbs, flour, and 1/4 cup cheese in a shallow dish.
Dip 1 rutabaga slice in milk, and dredge in breadcrumb mixture. Place slice in center of a greased 8-inch round cakepan.
Cut remaining rutabaga slices in half. Dip half of slices in milk, and dredge in breadcrumb mixture. Arrange in pan, allowing outer edges of rutabaga to fit around edge of pan.
Drain spinach well, pressing between layers of paper towels. Arrange over rutabaga. Sprinkle with bacon and remaining 1/2 cup cheese, and drizzle with 2 tablespoons butter.
Dip remaining rutabaga slices in milk; dredge in breadcrumb mixture. Arrange over cheese. Drizzle with remaining 2 tablespoons butter. Top with a sheet of aluminum foil; press an 8-inch round cakepan onto tart. Place tart in oven on preheated baking sheet, and top with a large heavy skillet.
Bake at 400° for 40 to 45 minutes.