In large skillet, melt butter over moderately high heat. Add onion and garlic. Cook, stirring, until softened but not browned, about 2 minutes. Reduce heat to low, cover and cook until onions are very soft, about 6 minutes. Add cream, wine, brandy, vinegar, mustard, salt and pepper. Increase heat to moderately high and boil, uncovered, whisking occasionally, until thickened, about 8 minutes.
Meanwhile, with melon baller, cut out as many half-spheres as possible from the two cucumbers. Add cucumber pieces to sauce, reduce heat to low and cook, stirring occasionally, until cucumber is slightly soft, about 5 minutes. Stir in all but 1 tablespoon each of parsley and dill. Remove from heat and cover to keep warm.
Heat oven to 400°. Pat salmon steaks dry with paper towels. In large ovenproof skillet, heat olive oil over high heat until smoking slightly. Add salmon to pan and saute for 1 minute on each side, until browned. Transfer skillet to oven and bake until salmon is just opaque throughout, checking for doneness around center bone, about 7 minutes. Remove salmon from oven and drizzle lemon juice over the top. Spoon reserved sauce onto large, warm serving platter and put fish on top. Sprinkle remaining 1 tablespoon each parsley and dill over salmon.