Heat oven to 375 degrees F. Coat a 2-1/2-quart baking dish with nonstick cooking spray.

Melt butter in medium-size skillet over medium heat. Add onions; cook, stirring occasionally, until softened and lightly browned, 12 to 15 minutes.

Thinly slice potatoes, either by hand or with slicing blade in food processor.

Spread third of potato slices in single layer, slightly overlapping, in bottom of prepared baking dish. Sprinkle with 1/4 teaspoon salt. Layer with half of sliced tomato. Top with half the onion, then half the shredded cheese. Continue layering with another third of potato, remaining tomato, onion and cheese, sprinkling potato layer with another 1/4 teaspoon salt. Top mixture with final layer of potato.

Whisk together half-and-half and flour in medium-size saucepan. Heat over medium-high heat, stirring constantly, just until warmed through, about 8 minutes. Remove from heat. Whisk in remaining 1 teaspoon salt, the herbed cheese, pepper and half the Parmesan. Pour over layered potato mixture. Cover dish with aluminum foil.

Bake in 375 degree F oven 40 minutes with baking sheet underneath to catch drips. Remove foil. Sprinkle with remaining Parmesan. Bake 30 minutes, until browned and potatoes knife-tender. Let stand 15 minutes. Makes 8 servings.