In a small saucepan over medium heat, combine all ingredients and bring to a simmer. Reduce heat to low and simmer until liquid starts to thicken, lowering heat if it bubbles too vigorously. Cook until dressing is thick enough to coat the back of a spoon, about 8 to 10 minutes. It will continue to thicken as it cools. Dressing may be made two days ahead; store in an airtight container and strain before serving.

Heat gas or charcoal grill to high at least 30 minutes.

Combine the espresso powder, ground mustard, cumin, Espelette pepper, brown sugar, and salt in a small bowl. Dry steak well with paper towels. Rub spices over steak.

Grill steak, flipping once. If the steak starts to char too much, move it to a cooler part of the grill. Use a meat thermometer to check for doneness (120°F rare; 125°F medium-rare; 130°F medium). Top steak with ½ cup grated cheese during the last minute of cooking; close grill cover to melt it.

Remove steak from grill when a few degrees below desired doneness temperature. Tent with foil and allow to rest for 10 minutes. Steak will continue to cook as it rests.

Meanwhile, prepare squash and fruit. Drizzle with olive oil and season with salt. Grill squash until well browned, 8 to 12 minutes. Grill grapes until softened, 5 to 7 minutes. Grill strawberries 3 to 4 minutes. Once cool enough to handle, chop squash into ½-inch pieces.

Slice steak against the grain into thin pieces. Combine steak, fruit, vegetables, and watercress on a large platter. Drizzle with dressing, sprinkle with remaining ½ cup cheese, and serve immediately.