Making the horseradish sauce. Combine the crème fraîche (or yogurt) with the grated horseradish, salt, and pepper in a bowl. Put the bowl in the refrigerator and allow the sauce to marinate for at least 30 minutes.
Cooking the quinoa. In a small skillet, sauté the onion and garlic in olive oil over medium-high heat until fragrant (about 2 minutes). Add the quinoa and toast it in the olive oil for about 1 minute. Add the stock and salt to the pan and bring mixture to a boil. Lower the heat to medium-low and let the quinoa simmer until it is tender (about 15 minutes). Spread the quinoa out on a plate and set it in the freezer for a few minutes to cool down.
Coring the tomatoes. Bring a pot of salted water to a boil. Take the cores from the two tomatoes. Flip them over and use a sharp knife to score a small x into the bottom of each tomato. Put them in the boiling water. When the skin starts to slip off, remove the tomatoes from the boiling water and submerge in a bath of ice water immediately. When they have cooled, peel the tomatoes, remove their seeds, and dice into small pieces.
Making the vinaigrette. Put the two vinegars, mustard, salt, and pepper in a blender and mix until well incorporated. Slowly, with the blender running, drizzle the oils into the blender until the vinaigrette has fully emulsifi ed.
Assembling the salad. Take the quinoa out of the freezer (it should be well cooled but not at all frozen). Add the diced tomatoes, orange pepper, and green onions to the quinoa and stir. Drizzle a small amount of the vinaigrette over the quinoa and toss the salad. (Save the reserved vinaigrette for later salads.) Lay the quinoa on a bed of the red leaf lettuce. Top with the horseradish sauce and the smoked salmon, if desired.