1. In a sauté pan over medium heat, melt a little of the butter. Add the thyme, lemongrass, leek, and ginger, and stir. Add the red wine and cook until wine is almost evaporated. Add demi-glace and reduce the sauce again. Strain sauce and set aside.
2. In a separate pan, melt some butter on medium heat. Season the wild boar pieces with salt and then add them to pan, turning heat up to high to sear the meat. Cook for 1-2 minutes per side, until the meat is medium rare. Remove boar from pan and set aside. In the same pan, add a little more butter, the portabello mushrooms, and salt and pepper to taste. Add the cognac and stir, deglazing the pan. Next, stir in the strained red wine sauce and cook until slightly reduced. Then swirl in about 1/2 tablespoon of butter. Add wild boar pieces to the pan, and cook until heated. Serve immediately.
Note: Wild game demi-glace can be purchased at retail companies that sell wild game meat. If you want to forgo the wild game demi-glace, you can make a sauce without it after sautéing the portabello mushrooms with butter, salt, and pepper, and cognac, add the thyme, lemon grass, leek, ginger, and red wine and reduce. Then, swirl in about 1/2 tablespoon of butter.