Warm Spinach Salad with Bacon, Tomatoes, and Walnuts

• 1/2 cup candied walnuts
• 6 slices bacon, cut into 1-inch pieces
• 1 red onion, halved and thinly sliced
• 1 pint (2 cups) grape tomatoes
• 1/3 cup white-wine vinegar
• 1 tablespoon sugar
• 2 to 3 bunches (1 1/2 pounds total) flat-leaf spinach, trimmed, cleaned, and dried
• Coarse salt and ground pepper
• In a large skillet over medium heat, cook bacon, stirring occasionally, until browned and crisp, 6 to 8 minutes. Transfer to a paper-towel-lined plate. Pour off all but 3 tablespoons fat from skillet.
• Add onion to skillet; cook, stirring occasionally, until crisp-tender, 5 to 7 minutes. Add tomatoes, vinegar, and sugar; simmer until liquid thickens slightly and tomatoes are heated through, 1 to 2 minutes.
• Place spinach in a large bowl, and pour hot tomato-onion mixture over; toss quickly to coat and wilt spinach. Season with salt and pepper. Sprinkle with walnuts and bacon.