Place a large skillet over medium heat and melt the butter. When the butter is melted, add the onion and cook for about 4 minutes, until translucent. Add the garlic and cook for 1 additional minute before adding the mushrooms.
Allow the mixture to cook over medium heat for about 10 minutes, stirring frequently until the mushrooms have softened and become dark brown around the edges and the onions have become a nice brown caramel color.
Once the mushrooms have cooked completely, season with ½ tsp of salt and stir to combine.
Add the frozen peas and toss to combine. Cook for about 5 minutes, until the peas just become hot. Remove from the heat and season to taste with salt and pepper. Serve immediately and enjoy!
Notes
We use petite peas. They are sweeter and crisper than sweet peas. But, you can use whichever you prefer. Depending on how sweet the peas actually are, tone up or down the salt level before serving.