Combine all ingredients for the marinade in a gallon bag and add the flank steak. Marinate at least 30 minutes.
Julienne (very thinly slice) your vegetables and toss in the rice vinegar, honey, salt, and pepper for the quick pickled veggies. Allow them to sit at least 20 minutes.
On a grill at medium-high heat (or on a grill pan on the stove), cook the steak. Cook to your personal preference - we did ours about 8 minutes per side. Remove from heat and allow to rest 10 minutes.
Toast your baguettes or rolls on the grill.
Once your meat is rested, slice it against the grain very thinly.
Assemble the sandwiches by layering the spinach, sliced meat, and quick pickled veg. Enjoy!