Cook pasta in a large saucepan of lightly salted boiling water until al dente, following packet instructions.
Heat the oil and butter in a large non-stick frying pan over medium heat. Add onion, garlic, oregano and marjoram. Sauté until the onion is softened, about 5 minutes. Stir in flour until thoroughly combined. Whisk in the stock.
Heat, stirring, until thickened, 2 to 3 minutes. Stir in peas and sun-dried tomato. Cook just until peas are tender, about 5 minutes. Remove from heat. Stir in 3 tbsp Parmesan.
Drain pasta, reserving ½ cup (125 ml) of the cooking water. Toss pasta with vegetable mixture and reserved water. Sprinkle with the remaining Parmesan.