The trick is to eat more. If you're low carb, then you are probably getting enough protein from animal sources, so what's missing?

Insulin is an anabolic hormone. Aside carbs, insulin release is stimulated by protein ingestion, so the solution is to add another meal. This should be fairly obvious because usually this requirement manifests itself as a voracious appetite, at least for me.

FWIW, I have gained 2.5 kg of fat and 7kg of lean muscle on a low carb diet in three months of resistance training with no cardio (Starting Strength method).