Heat oil in a large saucepan over medium-high heat.
Cook onion, garlic, thyme, bay leaf, parsley, and
peppercorns, stirring often, until garlic is deep
brown, about 5 minutes. Add wine, bring to a simmer,
and cook, scraping up any brown bits, until liquid is
reduced by half, about 4 minutes.
Add Parmesan rinds and 8 cups water; bring to a boil.
Reduce heat and simmer, stirring occasionally to
prevent cheese from sticking to bottom of pot, until
broth is flavorful and reduced by half, about 2
hours.
Strain through a fine-mesh sieve into a large bowl
(or an airtight container if making ahead).
Do Ahead- Broth can be made 4 days ahead. Let cool;
cover and chill.
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