Run frozen peas under hot water in a strainer to partially thaw.

In a large cast iron (or other heavy) skillet over medium heat, melt butter. Add onion and cook until softened, about 4 minutes. Add garlic and cook 30 seconds.

Stir in flour and cook 1-2 minutes, whisking constantly. Slowly whisk in milk and cream, whisking until smooth. Bring to a gentle simmer and cook until thickened, about 2 minutes.

Add peas, salt, and pepper. Cook and stir until peas are warmed through and sauce is creamy, about 5 minutes. *If your peas are still partially frozen when added, the sauce will seize up, that’s ok! Just keep stirring and cooking and the sauce will thin back out as soon as the peas are warm.

Taste and adjust seasoning. Serve immediately, garnished with green onion if desired.