Light a grill or preheat a grill pan. Grill the bell pepper until charred all over. Transfer to a bowl, cover with plastic wrap and let steam for 5 minutes. Peel the pepper and discard the skin, core and seeds, then slice 1/2 inch thick.
In a small bowl, mix the mayonnaise with the mustard, miso, lemon juice, garlic and cayenne pepper. Season the mayonnaise with salt and pepper.
Brush the asparagus and onion slices with the olive oil and season with salt and pepper. Grill over a moderately hot fire until the asparagus are crisp-tender and the onion slices are tender, about 5 minutes per side for each. Transfer the vegetables to a platter. Toast the bread slices on the grill.
Spread the garlic mayonnaise on 4 slices of toast and sprinkle with the basil. Top with the asparagus, onion and strips of red pepper. Close the sandwiches, cut them in half and serve.
One Serving 313 calories, 14.5 gm total fat, 1.8 gm saturated fat.