In a small non-reactive bowl, add the shallot and cover with the red wine vinegar. Season with a little salt and pepper and let rest on the counter while you prep the rest of the ingredients. The longer the shallots marinate in vinegar, the more tender and mellow they’ll become.
In a small bowl, whisk together the dressing ingredients: olive oil, white wine vinegar, dijon mustard, garlic, lemon zest, and sugar. Season with salt and pepper to taste. Set aside.
Add the butter lettuce to a large serving bowl. Drain the shallots & discard the red wine vinegar marinade. Sprinkle the shallots over the salad, followed by the radishes and the feta. Drizzle the dressing over the top and toss with salad spoons. Enjoy right away!