For dressing: Whisk together lemon juice, vinegar, mustard, salt, pepper, and garlic. Whisk in olive oil or yogurt and olive oil. Taste and adjust seasonings, set aside.
Scrub 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 heat and toss while hot in a bowl with salt and pepper to taste, onions, parsley, goat cheese, and dressing. Sprinkle sage over top and serve.