Cook the pasta in boiling salted water until al dente, about 11 minutes. Drain.
Meanwhile, heat the oil in a frypan and cook the onion until soft, about 5 minutes. Add thyme and salt and pepper.
At the same time, heat the milk in a large saucepan. When tiny bubbles start to appear stir in the sour cream and blue cheese. Stir until all is smooth over low heat. Add the onions, pasta and frozen peas and stir until heated through. Serve!