In a large saucepan, cook the sausage over medium-high heat until browned and the fat is rendered. Add the onions and bell peppers, and cook, stirring, until very soft, 4 to 5 minutes. Add the garlic and cook, stirring, for 30 seconds. Add the rice and cook, stirring, until opaque, about 1 minute. Add the stock, salt, and saffron, and bring to a boil. Reduce the heat to low, cover, and cook until almost all the water is absorbed, about 15 minutes. Add the peas without stirring, re-cover and cook until all the water is absorbed, about 5 minutes. Remove from the heat and let sit for 10 minutes before fluffing with a fork.