This is a delicious accompaniment to our Rib Roast with Rosemary-Garlic Potatoes.
Heat 2 tablespoons butter in a large skillet over medium. Add onion, and season with salt and pepper. Cook, tossing occasionally, until onion begins to soften, 2 to 3 minutes.
Add peas; cover skillet, and cook, stirring occasionally, until peas are warmed through and tender, 6 to 10 minutes. Remove from heat.
Add remaining tablespoon butter; stir until melted. Season with salt and pepper, and serve.