Melt butter with oil in heavy large pot over medium heat. Add onion and garlic; saute until onion is tender, about 6 minutes. Add flour; stir 2 minutes. Gradually whisk in stock and cream; bring to boil, whisking constantly. Cook until mixture thickens, stirring frequently, about 2 minutes. Remove from heat. Stir in spinach, cheese, sour cream and cayenne (spinach will wilt). Season with salt and pepper. Transfer dip to serving bowl. Serve warm with toasted baguette slices.