In a medium saucepan on medium, toast cumin seeds, 45 seconds. Remove from heat, add beans and ¼ cup water and roughly mash. Cook until heated through, 3 min. Adjust consistency with additional water as necessary so mixture is thick and saucy.
In a food processor, finely chop garlic and jalapeño. Add parsley and pulse to finely chop. Pulse in lemon juice and ¼ tsp salt to combine. Add oil and pulse once (mixture should not be emulsified); transfer to a bowl.
Fold tomatoes into parsley sauce. Serve spooned over warm fava mixture, with toast if desired.
