Fry the bacon until golden in a dry pan. Remove it from the pan and drain it on paper towels. Leave about 1 tablespoon of oil in the pan and add the spinach, onions and rosemary. Saute and stir until the spinach and onions soften.
Put the spinach and bacon in a food processor and blend. Season to taste with salt and pepper. Add milk and cream to get the desired consistency.
Season, and if necessary, return to the stove for a few minutes.