Potato Gratin for 50
12 pounds potatoes, peeled and sliced
1 large onion, thinly sliced
2 sticks butter
1/4 cup flour
4 1/2 tsp salt
1 gallon milk (combo milk, cream)
4 garlic cloves, halved
2-3 cups fresh grated parmesan cheese (or combo cheddar)
Grease two 12x20x2-inch baking pans.
Melt butter, whisk in flour and salt, stir until smooth, add garlic. Cook 5 minutes.
Gradually whisk in milk, heat until thickened. (remove garlic with slotted spoon before becomes too thick)
Layer 3 lb sliced potatoes in each pan. Top each with 1/4 of the sliced onion, 1/4 of the white sauce and 1/4 of the cheese. Reapeat, layering remaining potatoes, onion, white sauce and cheese.
Bake at 350°F for 1 1/2-2 hours.