Place the onion halves and thyme sprigs in a 2-inch deep baking tray. Pour the mushroom stock and half the butter over the onions. Bake in the oven for 30 minutes. Remove from the oven and let cool for 15 minutes.
Scoop out the center of the onions. Heat the rest of the butter and the olive oil in a saute pan over medium-high heat. Add the garlic and saute for 2 minutes. Then add the lobster mushrooms and cook for 5 minutes. Add the chopped parsley and season with salt and pepper. Fill the center of the baked onion rings with the lobster mushroom mixture.
To plate, divide the Creamed Spinach onto center of the 6 plates. Place the lobster mushroom filled onions onto the spinach. Finish by spooning the Bourguignon Trimming Sauce over top.
Over medium-high heat in a large saute pan, melt the butter. Add the shallots and cook until translucent. Add the spinach and cook for 4 minutes. Add the heavy cream and cook for another 3 minutes, until the cream has thickened. Season with salt and pepper.
Bourguignon Trimming Sauce:
In a medium saucepan over medium heat, render the bacon until crisp and golden brown. Add the flour and stir to make a roux. Then whisk in the mushroom stock and bring to a simmer. Cook until the mushroom stock has thickened, about 5 minutes. Remove from the heat. Add the pearl onions, tomatoes, parsley and green onions.