Cover the pan and simmer over low heat for 25 minutes.
Add the onions D and continue simmering until they are soft and the chops are tender, 10–15 minutes. Turn the chops occasionally and add more broth or water if the sauce gets too thick. (The cooking time will depend on the thickness of the chops. They should yield slightly when prodded with a two-pronged fork.)
Discard the bouquet garni.
Transfer chops to warmed plates.
Add to pan and bring just to a simmer.
Remove pan from heat.
Add to pan and stir to combine.
Spoon sauce over the chops and serve.
The fresh, piquant taste of mustard will turn bitter if overcooked. It should not be boiled.
For a modernist method, sous vide the meat and onions. Lightly brown each chop by deep frying at 200°C / 392°F for 40 seconds. Pay with paper towel to remove the oil immediately out of the fryer. (Consider freezing each chop for 30 minutes prior to deep frying.) The deep fry stage has multiple benefits: it begins to form a crust, uneven surfaces are plumped up and the meat holds it shape better when vacuum packed (no need to add oil to the vacuum bag). Vacuum seal and cook sous vide for 2 hours 30 minutes at 54°C / 129°F. When time to serve, remove from bag, pat dry and finish browning by deep frying again at 200°C / 392°F for another 40 seconds. Pat with paper towel to remove oil. Cook the onions at 83°C / 181°F for 1 hour 30 minutes and then pat dry with paper towel before deep frying at 180°C / 356°F for 3 minutes. Prepare the sauce in advance (hold off on the parsley until service) and hold in a vacuum flask to keep warm. Using 0.5 g of xanthan gum in place of the flour to thicken the sauce.