For the caramelized onions:
2 tablespoons vegetable oil
5 cups yellow onions, thinly sliced
1/2 teaspoon brown sugar
1/2 teaspoon salt
1 tablespoon sherry vinegar
For the white sauce:
2 tablespoons butter
2 tablespoons all-purpose flour
3 cups half and half
2 teaspoons seasoned salt
For the casserole:
2 pounds fresh green beans, trimmed
4 ounces grated Gruyere cheese
1/2 cup grated parmesan cheese
1/2 cup shredded mozzarella cheese
1 cup breadcrumbs
Pre-heat oven to 375 degrees F.
For the caramelized onions: In a large skillet on medium-high heat, add oil and onions and brown sugar. Cook, stirring every 4 or 5 minutes until they turn brown and sweet. Do not let onions turn too dark. Reduce heat if needed. Season with salt and sherry vinegar. Cook until vinegar has evaporated.
For the white sauce: On medium-low heat, melt butter in a medium sauce pan. Once butter starts to bubble, whisk in flour. Cook for 3 minutes while whisking. Slowly whisk in cold milk and seasoning salt. Simmer for 5 minutes until sauce thickens and any lumps disappear.
Bring a large pot of water to a rapid boil and blanche green beans for 3 minutes.
In a casserole dish, combine beans, caramelized onions, white sauce and Gruyere cheese.
Sprinkle parmesan, mozzarella and breadcrumbs over top of casserole.
Cover loosely with aluminum foil and bake for 30 minutes. Remove foil and continue to bake for 15 minutes.
Pro Tip: For easier prep, use thawed frozen green beans