Put onion, garlic cloves and chili (or jalapeno flakes) in food processor and finely chop (not paste though)
Heat oil in large frying pan over low heat
Add mustard seed, cumin seeds, and tumeric and saute for about 30 seconds
Add onion mix and saute on low just until they begin to soften, not brown
Add cabbage and fennel and toss until coated and yellow
Add garam, corriander, chili powder, salt and pepper
Stir to coat cabbage
Cook for 10 minutes, partially covered, occasionally stirring (you may need to add a splash of water every so often so the cabbage doesn't stick)
Add butter and adjust salt level, stirring to mix
Add a splash of lemon juice and toss just before serving