In a cast iron skillet, heat the oil over medium-high heat. Add sausage and cook for 4 to 5 minutes until brown, rendering some of the fat. Remove using a slotted spoon and transfer to a bowl.
Add onion and garlic to the hot skillet and sauté until fragrant, about 2 minutes. Add rice and to the skillet and toss to coat with sautéed onions. Add diced tomatoes, stock, cooked sausage, salt, saffron and stir gently. Bring to a simmer, reduce heat to low, cover and cook for 20 minutes.
Remove the lid, nestle shrimp and mussels into the rice. Put the lid back on and continue to cook for 4 more minutes or until shrimp or opaque and mussels have opened. Sprinkle with parsley and serve with lemon wedges.
Store leftovers in an airtight container in the refrigerator for up to 4 days.