Clean: Thoroughly wash the leek tops to remove all grit.
Combine: Add leeks, water, thyme, garlic, salt, and peppercorns to a large stockpot.
Simmer: Bring to a boil, then reduce heat and simmer for 30 minutes (or up to 1 hour for deeper flavor).
Strain: Remove from heat and strain the liquid through a colander.
Store: Use immediately, or keep in a sealed container in the fridge for up to a week, or freeze for 3 months.