Place baguette slices on an ungreased baking sheet; spread evenly with 1 tablespoon butter. Bake at 400° for 5 minutes or until crisp and golden. Let cool. Process in a blender or food processor until crumbs form. (This will yield about 1 1/2 cups crumbs.) Set aside.
Place squash, onion, and bay leaves in a Dutch oven with a small amount of water. Bring to a boil; reduce heat, and simmer, uncovered, 10 minutes or until squash is tender. Drain, discarding bay leaves. Sprinkle squash with salt and pepper.
Melt 6 tablespoons butter in a 4-quart saucepan over medium heat. Whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in milk; cook over medium heat, stirring constantly, until thickened and bubbly. Remove from heat; stir in seasoned salt and next 3 ingredients. Gradually stir about one-fourth of hot white sauce into yolks; add to remaining hot white sauce, stirring constantly. Stir in 1 cup cheese.
Combine squash and cheese sauce. Spoon into a greased 2-quart baking dish. Combine remaining 1/2 cup cheese and reserved breadcrumbs; sprinkle over squash. Bake, uncovered, at 350° for 40 minutes or until browned.
Note: If you can't find fresh yellow squash, substitute frozen sliced squash.