Heat the oven in the Convection mode 340° F (170 C)
Add 2 tbsp of the olive oil to a small heavy skillet and heat on low. Level 5 Induction or level 5 Gas cooktop. Add the minced garlic and sweat until the garlic begins to brown. Remove the pan from the heat and stir in the plant-based butter. Stir until the butter has completely melted then set aside.
In a large mixing bowl combine the panko breadcrumbs with the chopped spinach and parsley, melted butter mixture, pepper, chili flakes, nutritional yeast and tarragon to form a dough-like consistency.
Wipe any debris from the mushrooms with a damp paper towel, trim the stems and slice each mushroom in half lengthwise, then place them on the blue oven tray or a rimmed baking sheet. Sprinkle with the salt and drizzle with the remainder of the olive oil. Top the mushrooms with the breadcrumb mixture and press it firmly into the mushrooms.
Place the tray in the oven with the angled lip facing the door and bake for 20 - 25 minutes or until the mushrooms are golden brown and the topping is crispy.
Remove the mushrooms from the oven and finish with a little grated fresh horseradish, the Meyer lemon zest and juice.