Place thinly sliced red onion in a small bowl. Add white wine vinegar, warm water, sugar, and salt. Stir to combine, pressing the onions slightly so they submerge. Let sit at room temperature for at least 10 minutes (up to 1 hour) to mellow and lightly pickle. Drain any excess liquid before assembling the sandwich.

Quick guacamole (makes ~1/2 cup)

Halve the avocado, remove the pit, and scoop flesh into a bowl. Mash with a fork to a chunky consistency. Add lime juice, minced cilantro (if using), salt, and black pepper. Taste and adjust seasoning. Keep covered until ready to assemble to prevent browning.

Sandwich

Prepare roll

Slice the hoagie roll lengthwise, leaving a hinge if desired. If you prefer the roll warmed, toast lightly under a broiler or in a skillet for 1–2 minutes until slightly crisp.

Dress roll

Spread mayonnaise evenly on the bottom half of the roll and spread hot sweet mustard on the top half. Use the full tablespoon amounts or adjust to taste.

Layer ingredients

On the bottom half (mayo side), place the smoked turkey slices evenly across the length of the roll (about 6 oz). Lay the two slices of provolone over the turkey so they slightly overlap. Add the drained marinated onions evenly on top of the cheese. Spoon the guacamole in an even layer over the onions (about 1/2 cup). Arrange the tomato slices on top of the guacamole and sprinkle with 1/4 tsp salt and a few grinds of black pepper. Finish by scattering the sprouts over the tomatoes.

Finish

Close the sandwich gently but firmly. If you like, press briefly and slice in half. Serve immediately.