Light an outdoor charcoal fire or preheat a gas grill.
Heat 2 tsp oil in a large skillet over medium heat. Add onion and cook until soft and transparent, about 6 minutes. Stir in collard greens (still frozen) and 1/2 cup water or broth. Simmer for 12 - 15 minutes, adding more water or broth as needed. Stir in ginger and soy sauce, transfer to a serving plate and keep warm.
Rub salmon steaks with remaining 2 tsp oil. Season with a little coarse salt and a generous grinding of black pepper. Grill about 3 - 4 minutes per side, depending on thickness. Set salmon steaks on collard greens and serve.