Arrange 1 segmented red grapefruit on an oiled baking sheet; sprinkle with 2 tablespoons brown sugar and 1/2 teaspoon smoked paprika. Broil until browned, 4 minutes. Whisk 2 tablespoons rice vinegar and 1 teaspoon Dijon mustard in a large bowl; season with salt and pepper. Whisk in 2 tablespoons olive oil. Add the grapefruit, 5 ounces baby spinach and 1/2 cup chopped walnuts; toss.