Make the dressing first. Whisk together the lemon juice, vinegar, mustard, salt, pepper, and garlic. Whisk in the olive oil or the yogurt and olive oil. Taste and adjust seasonings, set aside. Scrub the potatoes and cut into 3/4-inch cubes. Steam above 1 inch of boiling water until tender but not mushy, about 10 to 12 minutes. (If you use white potatoes, check rather often, they get tender faster than Yukons or reds.) Remove from the heat and toss while hot in a bowl with salt and pepper to taste, the onions, parsley, goat cheese, and the dressing. Sprinkle the sage over the top and serve.