Serves 3-4; can be multiplied.
For the marinade:
1/3 cup reduced-sodium soy sauce
1/3 cup rice vinegar
2 Tbsp sesame oil
2 Tbsp mirin
1 Tbsp ponzu
1 tsp Sriracha, or more to taste
1 tsp minced ginger root
1 tsp minced garlic
1 tsp honey
For the beef:
3/4 lb thin-sliced beef sirloin tips
1/2 lb asparagus spears, ends trimmed
1 bunch scallions, trimmed to be the same length as the asparagus
In a casserole dish, whisk together all of the marinade ingredients. Place the beef and vegetables in a single layer, and turn several times to coat on all sides. Let sit at room temperature for 1 hour, turning everything 3 or 4 times.
Preheat a panini press, griddle, or grill to high heat.
Quickly cook the meat and vegetables; if the meat is thin enough, the cooking should take only 2 minutes. Do not overcook.
Serve hot or at room temperature.