Preheat the oven to 450°F (232°C).
Toss the vegetables and onion in olive oil in an ovenproof dish or baking sheet.
Roast the vegetables for 15 minutes, stirring once, then stir in the garlic and roast for 5 minutes more.
Place the vegetables and the bay leaf, peppercorns, parsley and thyme in a large stockpot with the water.
Simmer on low for 1½ hours.
Ladle the stock through a fine mesh sieve into heat-safe/freezer-safe containers.
Refrigerate for up to 1 week or freeze immediately for up to 3 weeks.