1. In a large (12-inch) nonstick or well-seasoned cast-iron skillet, heat olive oil over medium-high. Break up any stuck-together gnocchi and add to the skillet in an even layer. Cover and cook, undisturbed, until the gnocchi are golden brown underneath and unstuck from skillet, 2 to 4 minutes. Cook, stirring, until crisp on both sides, 2 to 3 minutes. If the gnocchi are burning instead of browning or the skillet looks dry, add more oil. Transfer to a bowl or plate.
2. Add the sausage to the skillet and break into small pieces. Cook, undisturbed, until sausage is browned, 2 to 4 minutes. Stir and cook until the sausage is cooked through, another 2 to 4 minutes.
3. Stir in the peas, mustard and ½ cup water and scrape up the browned bits on the skillet. (It may not look like a lot of liquid, but the peas will release some as they cook.) Simmer until the peas are cooked through, 2 to 4 minutes.
4. Add the browned gnocchi and the Parmesan; stir until the cheese has melted. Season to taste with salt and pepper, then stir in the herbs. Serve topped with more herbs, Parmesan and black pepper as desired.