I planted a full grown onion and has grown a stalk and a flower is the onion still good?

No, those bulbs are food for plants, like yolk in an egg, for chickens

I planted a full grown onion and has grown a stalk and a flower is the onion still good?
if you mean, can you still eat the bulb, you could without harm, but it has rooted (and is probably "mushy") and the flower will now produce "seeds"(in some varieties such as Egyptian walking onions they will produce small onion sets or bulbs) eventually the stalks will fall over in an attempt to plant their progeny in the surrounding ground. you can cut the stalks and use them like green onion tops anytime, more will come up.