Preheat your oven to 375°F (190°C). Line a baking sheet with foil for easy cleanup.

Warm the oil in a skillet over medium heat. Add mushrooms and let them cook, untouched, for 2 to 3 minutes to brown nicely.

Stir and cook until golden and the liquid evaporates. Add the garlic and cook for 30 seconds until fragrant.

Stir in spinach and artichoke hearts. If using fresh spinach, cook until it wilts. Remove from heat and cool for a few minutes.

In a mixing bowl, blend cream cheese, sour cream, mayo (if using), mozzarella, Parmesan, lemon zest and juice, salt, pepper, and red pepper flakes.

Fold in the mushroom-spinach-artichoke mixture until well combined.

Cut a long slit along the top of the baguette and gently hollow it out, leaving about 1/2 inch of bread around the sides and bottom.

Spoon the dip into the baguette, packing it in to avoid pockets of air. Sprinkle extra mozzarella on top.

Wrap the baguette loosely with foil and bake for 15 to 20 minutes until hot in the center.

Unwrap and bake another 5 to 8 minutes to get the top golden and bubbly.

Let it rest for 5 minutes, then slice into thick pieces. Garnish with parsley or green onions.