Melt the butter in a medium saucepan over medium-high heat. Add the onions and cook, stirring, for 3 minutes. Add the garlic and cook, stirring, for 30 seconds. Add the rice, salt, and pepper, and cook, stirring, until the rice is glassy, about 2 minutes. Add the stock and bring to a boil. Reduce the heat to low, cover, and cook until almost all the liquid is absorbed, about 10 minutes. Add the peas without stirring. Cover and continue to cook until the liquid is absorbed, about 4 minutes. Remove from the heat and let sit covered without stirring for 15 minutes.   Fluff with a fork. Add the parsley and mint and stir to combine. Adjust seasoning to taste and serve.