Preheat a grill to medium. In a medium bowl, squeeze 2 lemon wedges over the onion and toss well.
In a large bowl, whisk together the yogurt, 2 tbsp. mayonnaise and the juice from 2 more lemon wedges. Add the cucumber and 1/4 tsp. each salt and pepper and toss.
Rub the salmon on both sides with the EVOO and season lightly with salt and pepper. Add the salmon to the grill skin side down; cover and cook, without turning, until opaque on the edges and moisture appears on the surface, about 4 minutes. Lift the salmon off the grill (leaving the skin behind) and transfer to a plate.
Meanwhile, place the open buns on plates. Spoon half of the cucumber mixture onto the bun bottoms. Top each with a salmon fillet, some remaining cucumber mixture, some onion and some chips. Spread the remaining 2 tablespoons mayonnaise on the bun tops and place on the sandwiches. Serve with the remaining lemon wedges.