In a large stockpot over medium heat, add the oil. Saute onions, tomatoes, Fresno peppers and garlic for about 15 minutes, stirring frequently. Add the vegetable stock and squash. Bring to a boil, then reduce heat to a simmer.
Once squash is tender (about 15 minutes), place soup into a blender in small batches and puree until smooth. Return soup to the pot. Stir in salt, pepper and sugar. Sprinkle parsley on top before serving.