Gluten-Free Green Bean Casserole

Ingredients

• 2 pounds haricots verts or regular green beans (see note)
• Kosher salt and pepper
• 3 Tablespoons olive oil
• 1 small yellow onion, finely chopped
• 1 pound mushrooms, torn into bite-size pieces
• 2 Tablespoons gluten-free, all-purpose flour
• 2 cups vegetable stock or low-sodium broth
• 2 Tablespoons heavy cream
• 1 Tablespoon fresh lemon juice
• 2 teaspoons Dijon mustard
• 1 Tablespoon chopped fresh tarragon, plus more for garnish
• Store-bought or homemade crispy shallots, for garnish
• Italian parsley leaves, for garnish (optional)