Cook bacon in a medium skillet on medium heat, stirring occasionally, until crisp, 8 to 10 mins. Transfer to a paper-towel-lined plate with a slotted spoon; pour off all but 2 Tbsp. grease. Add onion. Cook, stirring occasionally, until tender, 4 to 6 minutes. Add sherry vinegar and Dijon mustard; cook, scraping up any brown bits, until incorporated, 1 minute. Remove from heat; whisk in olive oil. Arrange tomatoes on a platter and drizzle with vinaigrette. Top with bacon, blue cheese, and chives.