1. Cut off the base of the Brussels sprouts and remove any damaged leaves.
Bring a large pot of salted water to a boil.
Once the water is boiling, add the Brussels sprouts and let them cook for 5 minutes.
Drain them.
2. Empty the pot, refill it with salted water, and cook the Brussels sprouts for 10 to 15 minutes, depending on their size.
Check the doneness with a knife blade: it should easily pierce through.
Drain and set aside.
3. Peel the onion and finely chop half of it. In a small pot, melt the butter and let the onion become translucent for 2 to 3 minutes with a pinch of salt.
4. Add the Brussels sprouts, sauté them for 2 to 3 minutes over fairly high heat, then stir in the cream and mustard.
Mix carefully and let simmer for 2 minutes, just until the sauce coats the sprouts well.
5. Sprinkle with fresh thyme and serve immediately.