To make the balsamic reduction, pour balsamic vinegar in a heavy-bottomed medium-sized saucepan and set over medium-high heat. Stirring all the while, maintain a constant but gentle bubbling, edging the heat down a bit as needed. Cook for about 10 minutes, or until mixture has reduced by half. Reduction should be thick, but it should still easily drip off the back of a spoon. Set it aside while you grill the vegetables.
Prepare a hot grill.
Prep the romaine. Leave the core intact, but slice the reddish tip off the very end of the bunch. Next, working from end to top, halve the romaine. Immerse in water, and shake to dislodge any sand, paying close attention to the very base. Shake out and pat dry. Set aside.
Grill the garlic scapes over the hottest part of the grill, cooking each side for 1 to 2 minutes. Slice into 4-inch sections and set aside.
Grill the peaches on each side for 2 to 3 minutes. Use a sharp paring knife to slice out the pits and then cut each half in half (to make quarters).
Grill the romaine for 1 to 2 minutes on each side. Cut out the core and chop the leaves into bite-size pieces.
Arrange grilled vegetables on plates.
Next up, season both sides of the skirt steak with a generous pinch of sea salt. (Adding salt just before cooking leaches less moisture from the meat and makes for an excellent sear.) Cook for 3 to 4 minutes a side, let rest for 5 minutes, and then slice into 1/4-inch slices.
Lay several slices of steak on each plate and finish with a drizzle of balsamic reduction and several twists ground black pepper.