Cauliflower Soup with Chive Oil and Rye Crostini

INGREDIENTS

• 1/4 cup heavy cream
• 12 tablespoons (1 1/2 sticks) unsalted butter, softened, divided
• 1 large onion, minced
• 8 thin slices cut from Finnish rye bread (for crostini) or 4 slices pumpernickel bread, cut into 1/3" cubes (for croutons)
• 3/4 cup plus 1 tablespoon olive oil
• 1 large head of cauliflower (about 2 pounds), leaves discarded
• 2 sprigs rosemary
• 2 1-ounce bunches chives
• Kosher salt