Prepare 1 (12-oz.) package spaghetti according to package directions; drain. Sauté 1 large onion, chopped, in 2 Tbsp. hot olive oil in a medium skillet over medium-high heat 8 minutes or until tender. Stir in 1 (10-oz.) can mild diced tomatoes with green chiles and 1/8 tsp. salt. Cook 5 minutes or until liquid has almost fully evaporated. Whisk together 1 cup half-and-half and 1 Tbsp. all-purpose flour in a small bowl. Stir half-and-half mixture into tomatoes, and cook, stirring often, 5 minutes or until slightly thickened and bubbly. Serve sauce over hot cooked spaghetti.