Preheat oven to 450 degrees.
Put the carrot, celery, onion, garlic head and cremini mushrooms on a baking sheet. Drizzle the olive oil over top, and toss to coat. Add the tomato paste and toss again to evenly coat.
Spread vegetables in even layer, making sure the garlic head is cut side up.
Roast vegetables until browned, 20-25 minutes.
Add to vegetable broth the roasted vegetables, leeks, morels, bay leaves and thyme in a large Dutch oven. Bring to a simmer over medium high heat, then turn the heat down to low and simmer partially covered for 30 minutes.
Strain the stock through a fine mesh strainer into a container. Discard the solids from the strainer.
Stock can be pouring into quart-sized containers and frozen for up to 6 months