Creamy Cauliflower Soup With Cheesy Mustard Croutons

Name: Creamy Cauliflower Soup With Cheesy Mustard Croutons
Ingredients: 3 Tbsp. extra-virgin olive oil, divided, plus more for drizzling; 1 medium onion, chopped; 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, plus more; 3 garlic cloves, finely chopped; 1 large head of cauliflower (2½–3 lb.), florets broken into small pieces, core chopped; ¼ tsp. freshly ground white or black pepper, plus more; ¾ cup half-and-half; 1 Tbsp. Dijon mustard; 1 Tbsp. unsalted butter, melted; 5 cups country-style bread, preferably day-old, torn or sliced into 1 cubes; ½ cup finely grated Gruyère or cheddar; 1 Tbsp. plus 1½ tsp. fresh lemon juice