In a large saute pan, melt butter and olive oil over medium heat and stir to combine.
Add onions and garlic and cook for about 5 to 10 minutes just until the onions have become translucent. Using Instant Pot, use the saute feature. Otherwise, on stovetop, heat over medium high heat.
After onions have become translucent, add tomatoes, sugar, salt, oregano, and red pepper and stir well.
Cover and heat over low heat for 1 hour on stovetop OR using manual pressure, cook for 20 minutes in Instant Pot.
Remove from heat, pour into blender and blend until smooth. If using Instant Pot, use manual release. Instead of blender, I like to use an immersion blender (ie, a stick blender)
Stir in milk or cream if using and serve.
Garnish with croutons, pesto, or grated cheese if desired