Preheat oven to 350°F (175°C).
Thaw the frozen peas and set aside.
In a medium skillet, melt butter over medium heat.
Add onion and garlic; cook for 1–2 minutes until softened and fragrant.
Stir in flour and whisk well. Cook for 1–2 minutes to remove raw flour taste.
Gradually whisk in half & half, stirring until the sauce thickens and coats the back of a spoon.
Remove from heat.
In a large bowl, combine peas, mushrooms, cheddar cheese, and the creamy mixture.
Mix until well combined.
Transfer to a greased 9×13 baking dish and spread evenly.
Bake for 20 minutes.
Remove, sprinkle fried onions on top, and bake an additional 5 minutes.
Serve warm and enjoy this delicious Green Pea Casserole.