Bring 2 ½ cups of water to a boil, then add the demi-glace. Otherwise, use vegetable broth. Keeping a handful of raw peas aside, cook the rest in the boiling liquid for 5 to 7 minutes, depending on their size. While the peas cook, boil the pasta according to package directions in generously salted water. (For pappardelle, this is for seven to eight minutes.)
Place the peas and 150ml of their cooking water in a blender and mix until smooth, adding the basil and more broth if needed to produce a thin, flavorful sauce. Drain the pasta, return to the pot and drizzle with a drizzle of oil to prevent them from sticking. Pour in three-quarters of the pea sauce and stir. Check the seasoning. Divide between two or three soup plates and pour in the rest of the sauce.
Break the ricotta into large chunks, sprinkle it over the pasta with the reserved raw peas (but thawed or slightly warmed), pepper, and serve.