Heat vegetable oil in a Dutch oven or large saucepan.

Add chopped sweet onions and cook, stirring occasionally, until golden, about 10 minutes.

Add sweet potato and russet potato cubes and stir to coat with oil.

Sprinkle with salt, pepper, and cayenne pepper.

Pour in the water and heat to boiling.

Reduce heat and simmer uncovered until potatoes are very soft, about 30 minutes.

Puree about three-quarters of the potatoes in batches in a food processor or blender.

Return the pureed potatoes to the soup.

Crush any remaining large chunks with the back of a wooden spoon.

Stir in diced Italian tomatoes and simmer for 15 minutes.

Crumble cooked bacon over the soup and serve hot.