For the caramelized onion:
Heat olive oil in a medium frying pan and add sliced onion; sprinkle liberally with salt. Cook over medium heat for 30 minutes, stirring occasionally, until softened and golden. Lower heat and add a splash of water if onions start to dry out.
For the quick olive tapenade:
In a food processor, combine all tapenade ingredients and process until a textured mixture forms.
For the marinated beet:
Shred the beet finely using a hand grater. Whisk together the red wine vinegar, water, sugar, mustard, and a liberal amount of salt and pepper. Let the grated beets marinate in the marinade until ready to assemble the sandwich.
For the sauteed zucchini:
Once onions are done cooking, saute the zucchini in olive oil in the same frying pan over medium heat, laying the strips in a single layer in the pan. Cook on each side until brown--this will take several batches.
_For the sauteed thyme mushrooms:
Once the zucchini is done, saute mushrooms in olive oil, thyme and salt in the same frying pan over medium heat.
Slice the top third of your bread boule, and then hollow the center using your hands, leaving about an inch border around all sides and making sure not to punch through any surfaces. Hollow out the top third of the loaf as well.
Stir the thyme into the goat cheese and spread over the interior of the bread boule. Layer the rest of the cooked vegetables into the bread boule until you reach the top. Place the top third of the loaf back onto the sandwich. Wrap sandwich tightly in plastic or foil and place a heavy object on top to press it at room temperature for around 4 hours, or up to overnight in the fridge. Slice into wedges (recommended) or slices (a bit trickier) when ready to serve.