Quiche Lorraine Baguette

Ingredients

• 1 tablespoon unsalted butter
• 1 tablespoon olive oil
• 1 small onion, sliced
• 3 garlic cloves, minced
• 1 ¼ cups (4 ounces) sliced fresh mushrooms
• 1 teaspoon kosher salt, plus more for seasoning
• ½ teaspoon freshly ground black pepper, plus more for seasoning
• 5 eggs
• ¼ cup heavy cream
• 1 teaspoon Dijon mustard
• 5 slices deli ham
• ½ (10-ounce) package frozen spinach, defrosted and squeezed of excess liquid
• ½ cup (2 ounces) shredded gruyere or Swiss cheese
• ½ cup (2 ounces) shredded cheddar cheese
• 1 baguette (4×16-inch or 3x18-inch)