Preparation

Make the yogurt sauce

• In a small bowl, combine the yogurt, shallot, pickles, ketchup, Worcestershire, paprika, and hot sauce. Season to taste with salt and pepper. Set aside until needed, or store in an airtight container in the refrigerator for up to 5 days.

Make the mushrooms and spinach

• Position a rack in the center of the oven, and heat the oven to 400°F. Line 2 large rimmed baking sheets with parchment.
• Toast the coriander and mustard seeds in a small skillet over medium heat, stirring often, until fragrant, 2 to 3 minutes. Transfer to a spice grinder and let cool. Add the smoked paprika, granulated garlic, and sugar. Grind until the whole spices are cracked and coarsely ground. (Alternatively, grind in a mortar and pestle.)
• Put the mushrooms in a large bowl, and sprinkle all over with the spice mixture. Season to taste with salt and pepper, and toss with the Worcestershire and 4 Tbs. of the oil. Spread the mushrooms on the prepared baking sheets. Roast until the mushrooms are caramelized and all of the liquid has evaporated, switching the pan positions and stirring halfway through, about 25 minutes total. Transfer to a large bowl.
• Meanwhile, put the remaining 2 Tbs. oil and the garlic cloves in a large nonstick skillet. Cook over medium-high heat, stirring often, until the garlic is golden and fragrant, 3 to 4 minutes. Discard the garlic, and add the spinach. Season with salt and pepper. Cook in batches, tossing occasionally, until the spinach is wilted. Continue cooking, stirring often, until all of the liquid has evaporated and the spinach has darkened, 4 to 5 minutes total. Transfer the spinach to the bowl with the mushrooms, and toss to combine. Season to taste with salt and pepper. If making ahead, store the mushrooms and spinach in an airtight container in the refrigerator for up to 1 day.

Make the sandwiches

• Position a rack in the upper third of the oven, and set the oven to broil, and position a rack in the upper third of the oven. Line 2 large rimmed baking sheets with foil, and coat them lightly with cooking spray.
• In a medium bowl, stir together the sauerkraut and caraway seeds.
• Split the rolls in half horizontally, keeping them attached on one end so that they open like a book. Spread butter on the outside of each roll. Put the rolls, opened, on the prepared baking sheets. Place a slice of cheese on each roll. Divide the mushroom and spinach mixture among the rolls. Spoon about 2 Tbs. of sauerkraut on each sandwich, followed by about 2 Tbs. of the yogurt sauce (you won’t need all the sauce). Top each sandwich with another piece of cheese, and then close the rolls. Press down on each sandwich with the palm of your hand. (The sandwiches can be prepared to this point, and then wrapped in plastic. Store in the refrigerator for up to 1 day.)
• Slide one pan into the oven on the prepared rack. Broil the sandwiches until golden and toasted, rotating the pan halfway through, about 2 minutes. Remove the pan from the oven, flip the sandwiches over, press down with a spatula, then continue broiling until the bread is toasted on the other side and the cheese has melted, about 3 minutes longer. Repeat with the second pan of sandwiches. Cut the sandwiches in half, and serve with the remaining sauce on the side.