Whisk 2 tablespoons lemon juice, 2 tablespoons yogurt, ½ teaspoon mustard, the grated garlic, ¼ teaspoon pepper and ¼ teaspoon salt together in a large bowl.
While whisking constantly, slowly stream in ¼ cup oil until combined.
Add rinsed beans, 3 cups spinach, halved tomatoes, sliced shallots and 3 tablespoons dill; gently toss until evenly coated.
Sprinkle with ½ cup feta and ¼ cup walnuts.
