Put onion, garlic, ginger & 1/4 cup water into blender & blend till smooth.
Heat oil in skillet over medium heat. Add cumin & fennel. After 20 seconds or so, add the paste from the blender & the turmeric.
Cook for 20 minutes, stirring frequently. Add the coriander & fry stirring for a minute. Add tomato sauce & cook stirring for 1 minute. Turn off heat.
Wash okra & wipe with paper towel. Slice a few pods at a time into 1/4" rounds.
Turn the heat to medium again under the skillet. Add the okra, salt, garam masala, lemon juice & 6 tb hot water. When the onion paste is bubbling, cover, reduce heat to very low & cook for 35 minutes. Stir every ten minutes or so.
Madhur Jaffrey, "An Invitation to Indian Cooking"