Preheat oven to 425 degrees. Wash and dry all produce. Trim and thinly slice cucumber into rounds. Thinly slice jalapeño into rounds, removing ribs and seeds for less heat. Trim and halve mushrooms (quarter any larger mushrooms, if needed). Trim and peel carrots; grate on the largest holes of a box grater until you have 1/4 cup (1/2 cup for 4 servings). Cut remaining carrots into sticks (like fries; ours were 3 inches long and 1/3 inch thick).

In a small bowl, combine cucumber, jalapeño, vinegar, 1/2 tsp sugar (1 tsp for 4 servings), and a big pinch of salt. In a separate small bowl, combine mayonnaise, sour cream, and as much sriracha as you like. Season with salt.

Toss carrot sticks on a baking sheet with a drizzle of oil, salt, and pepper. Roast on top rack until browned and tender, 15-20 minutes. Remove from sheet. Meanwhile, in a medium bowl, whisk together tempura mix, 1/3 cup water (2/3 cup for 4 servings), and 1/2 tsp salt (1 tsp for 4). Stir mushrooms into batter until fully coated.

Heat a 1/3-inch-layer of oil in a large pan over medium-high heat. Once oil is hot enough that a drop of batter sizzles when added to the pan, use tongs to add coated mushrooms in a single layer. (You may need to work in batches.) Cook until golden brown, 3-4 minutes on the first side and 1-2 minutes on the second side. Transfer to a paper-towel-lined plate. Season with salt.

While mushrooms cook, slice baguettes lengthwise, stopping before you get all the way through. Place cut sides up on baking sheet used for carrots; toast on top rack until golden, 3-5 minutes.

Spread insides of baguettes with sriracha sauce; fill with as much pickled veggies, grated carrots, and tempura mushrooms as you like. Pick cilantro leaves from stems; use to garnish sandwiches. Divide sandwiches between plates. Serve with carrot fries and any remaining mushrooms and pickled veggies on the side.