Preheat oven to 375 degrees F.
Mix marinade ingredients. Cover ostrich pieces and marinate for 1 hour. Remove ostrich pieces from marinade and season each side. Grill for 8 to 12 minutes. To make criss-cross marks on meat, grill 2 to 3 minutes then turn meat on a 90 degree angle and grill another 2 to 3 minutes. Flip meat and repeat process on other side. Remove from grill and allow to rest for 3 to 4 minutes before slicing.
Heat first 2 tablespoons of olive oil in a large saucepan. To release their flavours sauté quartered celery, carrots and cooking onion, garlic and bay leaves for 1 to 2 minutes. Add drained lentils to the vegetables. Add enough water to the pot to come 1 inch above lentils. Cook for 15 to 20 minutes until lentils are tender but not mushy. Reserve 1/2 cup (125 ml) cooking liquid. Drain off the rest of the water. Spread lentils and vegetables out on a baking tray to cool. Discard vegetables, bay leaves and garlic.
In an oven proof sauté pan, brown pearl onions in second 2 tablespoons olive oil. Place in oven and roast until golden. Turn every 5 to 10 minutes. Add wine to pan and put back in oven. Cook until wine is reduced and is coating the onions. Remove from oven and add lentils, thyme and salt and pepper. Add the butter and enough of the reserved cooking liquid to make a coating over the lentils.
In a saucepan add wine, chicken stock and shallots. Simmer until liquid reduces to a third of a cup. Add the cream and simmer 5 to 10 minutes. Whisk in the homemade mustard. Check seasonings.