While making homemade stock does add an extra step, the big flavor it produces is well worth it. You can make the stock in advance and freeze until ready to use, if desired.
Heat oil in a large Dutch oven over medium-high heat. Add onion and carrot; sauté 8 minutes or until onion begins to brown.
Remove roots, outer leaves, and top 6 inches from leeks. Rinse well and drain. Reserve remaining portion for another use. Chop leek outer leaves and tops. Add roots, chopped leek, mushrooms, and remaining ingredients to onion mixture; bring to a boil. Reduce heat; simmer until reduced to 6 cups (about 30 minutes). Strain stock through a sieve over a bowl; discard solids.