why the taste is bitter?

Question

i often cook bok-choy or romaine lettuce and always stir-fry it with garlic, shallot, oyster sauce, salt and pepper, but somehow it's still bitter, how does it happen?

Answer 1

garlic & green leafy vegetables contain copper which makes makes food taste bitter at 10 mg/l or less.
well... reducing amount of pepper and the oystor sauce may help...

Answer 2

It is a bitter vegetable, as is oyster sauce. Hit it with sherry, and a bit of sugar before you add the oyster sauce.