We used orange tomatoes for their beautiful color, but red tomatoes make a soup that's just as delicious.
In a large stockpot, heat the oil over medium-low heat. Add onion and garlic; cook, stirring occasionally, until onion is translucent and very tender, about 15 minutes. Stir in marjoram, tomatoes, and sugar; season with salt and pepper.
Raise heat to medium-high, and bring to a boil. Reduce heat to medium, and simmer until liquid is reduced by one-quarter and has thickened slightly, 15 to 20 minutes. Remove from heat. Using a slotted spoon, remove marjoram sprigs. Let soup cool 10 minutes.
Pass soup through a food mill set over a bowl to puree; discard solids. Season with salt and pepper. Serve garnished with marjoram sprigs if desired.