Preheat oven to 400°F.
Place cauliflower in a Dutch oven with stock; bring to boil. Add drizzle of olive oil, season with salt and pepper, cover pot and cook until very tender. Mash then purée with an immersion blender with the Boursin and Parm cheese. Reserve.
Heat a large, cast-iron or deep skillet over medium-high heat. Add oil and when hot, add mushrooms and brown. Add meat and brown and crumble; add bay, rosemary, onions, celery, garlic, allspice, salt and pepper. Partially cover and sweat vegetables 7-8 minutes. Add tomato paste and stir 1 minute then add stock and reduce heat to simmer to thicken a bit.
Transfer meat to a casserole dish and top with even layer of cauliflower purée. Top with cheddar cheese.
Bake the casserole until golden brown on top and bubbly at edges.