Preheat the oven to 400°F (200°C). Cut off the top of the head of garlic and wrap it in foil. Drizzle olive oil over the top and sprinkle with a pinch of salt, pepper, and fresh parsley. Roast in the preheated oven for 20-30 minutes, or until the garlic cloves have softened.
Grill the chicken tenders for about 3-4 minutes per side, then set them aside to cool.
Chop the onions and mushrooms. Sauté the onions in 3 tablespoons of butter until tender. Add the mushrooms to the pan and cook until the mushrooms release their moisture and the onions start to brown. Remove from heat.
Cut open the sourdough loaf and remove the bread from inside, leaving about 1 inch of crust/bread around the sides. Set aside the excess bread.
In the bottom of the bread bowl, spread half of the tzatziki sauce, followed by the chopped yellow peppers and tomatoes.
Next, add the sautéed onions and mushrooms, roasted garlic cloves, and grilled chicken tenders. Top with fresh spinach. Spread the remaining tzatziki sauce on the lid and press it down as close to the cut as possible.
Wrap the loaf in parchment paper and then tightly wrap it in foil. Place it in a location where weight can be added on top. Apply 40-60 lbs of pressure for 4-6 hours. The longer it's pressed, the better the results.
(Optional) After the 4-6 hour period, unwrap the sandwich and place it, still wrapped, in the oven at 375°F (190°C) for 25-30 minutes to melt the cheeses.
When ready, remove the wrapping from the loaf and cut it into individual slices. Serve alongside oven-baked corn on the cob and enjoy, preferably outdoors!