Ina Garten's Creamy Tomato Bisque

Ingredients

• 1/2 tsp. saffron threads
• Kosher salt and freshly ground black pepper
• 3 tbsp. unsalted butter
• 1 1/2 c. chopped yellow onion
• 3/4 c. chopped shallots
• 1 tbsp. olive oil
• Grated Parmesan and chopped fresh flat-leaf parsley, for garnish
• 1 c. chopped leek, white and light green parts only
• 2 c. heavy cream
• 4 c. whole milk
• 1/4 tsp. crushed red pepper flakes
• 3 cans crushed tomatoes