Heat oil over medium-high heat in a large skillet. Add onion and cook 5 minutes, stirring occasionally. Add red bell pepper, carrot and garlic, and cook 5 to 8 more minutes, or until vegetables are tender, but not mushy. Stir in flour, black pepper, paprika, mustard, red pepper flakes and sage; cook 1 minute, stirring constantly. Gradually add milk, stirring after each addition. Cook 3 to 5 minutes, whisking constantly, until gravy comes to a gentle boil. Remove from heat, and stir in ranch dressing and drained lentils.