Preheat Oven to 400 degrees F.

Cut the cauliflower into florets; toss on a large rimmed baking sheet with 2 tablespoons of the olive oil; season with kosher salt and freshly ground black pepper. Roast, tossing occasionally, until almost tender, 30 to 35 minutes. Set aside until needed.

Meanwhile, in a large heavy pot or Dutch oven, heat the remaining olive oil. When the oil is hot, add the onions and thyme; saute over medium-low heat for 15 to 20 minutes, until translucent. Add the garlic and saute for about 2 to 3 more minutes. Turn off the heat and remove the sprigs of thyme.

While the cauliflower and onions cook, parboil the potatoes, carrots, and celery by adding them to In a medium saucepan with cold water to barely cover. Cover and bring to a boil over high heat. Reduce the heat to medium; cook, covered, for about 10 to 15 minutes or until you can easily pierce the potatoes with a fork. Drain the potatoes carrots and celery in a colander.

Using the same saucepan used to parboil the vegetables, heat the vegetable stock, but do not boil. Add the roasted cauliflower and half of the cooked onions to a food processor or blender. Pulse several times, then gradually add about ½ cup of the vegetable broth at a time to the cauliflower, scraping the sides as needed, until the cauliflower becomes a smooth puree.

Remember, you don't want to make it too watery, so it is better to add small amounts of the vegetable broth at a time. The longer you blend, the more silky and smooth the sauce becomes. Add more liquid if your sauce is too thick. When you reach the desired consistency, taste and season with salt, pepper, to taste.

Add the mushrooms to the remaining onions in the heavy pot or Dutch oven over medium-low heat. Cook, stirring occasionally until the mushrooms are tender. Add the potatoes, carrots, celery, and carefully pour the cauliflower sauce into the pot with the vegetables. Sir in the peas and heat through; about 5 to 7 minutes. Turn off heat.

Roll out the pie crust or puff pastry, place the ramekins on the sheet of pastry dough, so that there is about a 1-inch parameter around each ramekin. Using a knife, cut circles or squares large enough to fit over the ramekins.

Spoon the mushroom filling into each of the ramekins, cover the top of the ramekin completely folding the dough over the edges pressing it against the ramekin for each.

Using a fork, pierce the top of each pot pie, so that the steam will escape without making the crust rise.

For a prettier crust, I like to brush the dough with egg wash, (1 egg + 2 tablespoons water, beaten) on top of the crust to make them golden brown, when baked, but this step is completely optional.

Place the ramekins on a baking sheet and into a 400 degree F oven. Bake for 20 to 25 minutes, or until the tops are golden brown, and/or puffy if using puff pastry.

Cool 10 minutes before serving and enjoy!