1. Cut the meat into 2 to 3 centimeter cubes.
2. Roughly chop the onions.
3. Melt the butter and sweat the onions in it for 10 minutes to soften them (low heat, covered), then set aside.
4. Turn the heat to high and add the meat to the pot, stirring regularly (do not cover). The meat should brown on all sides and release some juices.
5. Remove the pot from the heat and transfer the meat to a dish, keeping the juices in the pot.
6. Dissolve the brown sugar in the meat juices and bring to a boil over high heat to reduce by half.
7. Once reduced, reduce the heat to low and add the onions back to the pot, mixing them with the 'syrup'. Add the meat and mix again, then pour in the beer and lightly salt.
8. Carefully cover the entire surface with the 'mustard-flavored' gingerbread.
9. Simmer covered for 3 hours without stirring, until the gingerbread is melted.
10. Adjust the seasoning if necessary during the simmering process.
11. If after three hours the sauce is still too liquid, continue simmering with the lid partially open. The sauce should be slightly sticky on the surface but still liquid underneath, and should not burn at the bottom.
12. Serve with fries.