Summer Squash and Onion Cheesy Casserole

Ingredients (9)

• + 1 ½ tablespoons butter, divided add to pantry
• + 3 summer squash, trimmed and cut into bite-size pieces add to pantry
• ✓ 3 zucchini, trimmed and cut into bite-size pieces
• + salt and ground black pepper to taste add to pantry
• + salt and ground black pepper to taste add to pantry
• + 3 onions, chopped add to pantry
• + 1 pound shredded sharp white Cheddar cheese add to pantry
• + ½ pound shredded Cheddar cheese add to pantry
• + 3 tablespoons sweetened condensed milk, divided add to pantry