Step 1: Gently snap off the tough ends of the asparagus. The spears should naturally break where they need to.
Step 2: Fill a large bowl with ice water and set it aside.
Step 3: Rinse the asparagus and then place them in the rack of a steamer pot, with the water level just below it. Sprinkle with salt and over high heat, bring the water to a boil and cover. Reduce the heat to low and steam until tender, about 4 minutes.
Step 4: Immediately add the steamed asparagus to the bowl of ice water and let it sit for a couple of minutes. (This will stop the cooking and keep the color bright.)
Step 5: Remove the asparagus from the water and spread it out on a double layer of paper towels to dry.
Step 6: Cut the crust off of the bread slices. (Save them to make bread crumbs for another use!)
Step 7: Spread a bit of the mustard on each slice, dividing it evenly amongst them.
Step 8: Cut the asparagus spears in half, horizontally.
Step 9: Place a few slices of the cheese on half of the bread slices, followed by about 8 asparagus-spear halves.
Step 10: Add a couple more slices of the cheese on top of the asparagus.
Step 11: Close each sandwich with the top bread slice and spread a thin layer of butter on it.
Step 12: To grill the sandwiches, you can use a sauté pan, a grill, or a panini press (following the manufacturer's instructions). For the sauté pan, melt a bit of butter on the bottom and place it over medium-high heat. Add the sandwiches, dry side down, and when the bottoms are golden, about halfway through the cooking process, gently flip them over. Press down on the sandwiches from time to time with a flat-bottomed metal spatula. Remove them from the pan once the cheese is melted. (If the bottoms are becoming too brown, turn the heat down.)
Step 13: Let them cool for a few minutes, and then cut them into fourths.
Step 14: Add a toothpick to each one and serve them on a platter. Lovely!
More Asparagus Recipes: