Season venison medallions with salt and pepper. Add olive oil to a hot skillet. Sear medallions over very high heat, about 3 minutes. Do not over cook. Leave the medallions medium rare for optimal tenderness. Remove from pan to rest in a warm place.

Add the onion and garlic clove to the pan. Sauté for several minutes until the onions become transparent. Remove and discard the clove of garlic. Add the mushrooms and sauté for 1 minute. Deglaze the pan with marsala wine and demi glace.  Add the Herbes de Provence, tomatoes and squash. Simmer for 1 minute. Add the venison medallions to the pan, just long enough to warm.