In a medium saucepan over high heat, bring 2 1/2 cups of water to a boil. Add the frozen peas, and boil for 1 minute. Stir in the couscous and butter; turn off the heat. Cover the pan and let rest for 5 minutes.
Add the mint and lemon juice to the couscous and fluff with a fork. Taste and season with salt and pepper, if necessary.