As potatoes roast, start the sauce. Heat 1 tsp. olive oil in a medium pan over medium heat. In a small bowl, whisk together bouillon cube with 1 ½ cup warm water until dissolved. Add remaining garlic and onion to pan and cook until translucent, about 4 minutes. Sprinkle with flour and stir to coat. Add bouillon liquid, tomato sauce, white vinegar, paprika, and red-pepper flakes to taste. Bring to a boil, reduce to a simmer, and let cook until slightly thickened and aromatic, about 30 minutes. Before serving, stir in any of the juices from the portbello roasting. Salt and pepper to taste. Set aside and keep warm.