Crack brittle shells from tamarind pods; peel off shells and fibers.

Put tamarind fruit and 2 cups water in a medium saucepan, cover, and bring to a boil.

Reduce heat and simmer for 5 minutes.

Remove from heat and let stand, covered, for 20 minutes, stirring occasionally.

Strain mixture into a bowl, pushing pulp through; discard contents of strainer.

In another medium saucepan, saute chopped onion in olive oil until golden, about 6 to 8 minutes.

Add tamarind pulp and remaining sauce ingredients (chopped canned chipotle chiles, ketchup, packed light brown sugar, kosher salt, pepper, pinch of ground cloves or allspice).

Bring to simmering over medium-high heat.

Reduce heat and simmer, partially covered, for 15 minutes to blend flavors, stirring occasionally.

Puree sauce in a blender until smooth.

Chill for up to 3 weeks.