Step 1

Heat oil in Instant Pot on Sauté setting (if using a manual pressure cooker, heat over medium-high). Add onions, cut sides down, and cook, undisturbed, until golden brown, about 3 minutes. Add celery, carrots, mushrooms, and garlic and toss to combine and coat with oil. Reduce heat to medium and cook with lid askew, stirring occasionally, until onions and mushrooms start to release some liquid and vegetables are browned in spots, 5–6 minutes. Stir in parsley, bay leaves, and peppercorns, then pour in 3 quarts water.

Step 2

Lock lid and cook stock on high pressure 40 minutes, then release pressure manually.

Step 3

Strain stock through a fine-mesh sieve into a large bowl; discard solids. Add salt (if using), stirring to dissolve. Let cool.

Step 4

Do Ahead: Stock can be made 4 days ahead. Cover and chill, or freeze up to 6 months.