Why does my cheese sauce taste like flour/paste?
It seems like you are not cooking the roux. Even a white roux should be cooked. A roux made with butter froths when it is cooked, after that you can add the milk. You can try adding it gradually, if it works for you, I find that dumping it at once and stirring vigorously is better for me - else the first small amount of milk gels the roux into too-hard lumps which the next portion of milk doesn't dissolve. Also, "until thickened" is a bit of a hit-or-miss. I wait until I see bubbles of cooking - not the rapid small bubbles of boiling water, but big, slowly rising bubbles plopping here and there, only then I stop cooking the sauce.