3 TBSP plus 1 tsp olive oil, divided
2 tsp Dijon-style mustard
2 tsp balsamic vinegar
2 tsp reduced-sodium tamari
4 three-inch-wide, half-inch-thick slices eggplant, from one small globe eggplant
1 sweet onion, peeled and cut into half-inch-thick slices
fine sea or kosher salt
freshly cracked black pepper
1 tsp lemon juice
baby arugula leaves
1/4 cup prepared hummus
In a shallow dish that fits the eggplant slices snugly in one layer, whisk together 3 tablespoons oil, mustard, vinegar, and tamari. Place eggplant in marinade in a single layer. Marinate 10 minutes, flipping halfway.
Meanwhile, heat a closing countertop electric grill. Toss onion slices with remaining teaspoon oil with a generous pinch of salt and freshly cracked black pepper, to taste. Place in a single layer on the hot grill, close the lid, and grill 6-7 minutes, until fragrant, with deep golden grill marks—rotate the slices after a few minutes so that you get sear marks running in both directions across the surface of the onion. When done, carefully transfer (tongs work best) grilled onions to a food processor bowl.
Give eggplant one last turn to coat generously with marinade and carefully place on hot grill in a single layer. Close lid and cook until tender and just a bit charred, 3-4 minutes—it will cook quickly since by this time, the grill pan is quite hot. Remove from grill.
While eggplant cooks, chop onions finely in processor. Stir in lemon juice and set aside.
Cut baguette in half lengthwise and cut off four six-inch segments. Divide onions among the bottom half of loaves. Cut each eggplant round in half and place two half-moons side by side atop the onions. Add arugula. Spread each top half of loaves with a tablespoon hummus. Sandwich and serve at once.