In a large glass dish, whisk together the soy sauce, white wine vinegar, canola oil, garlic, smoked paprika and pepper.
Add the flank steak and turn to coat with the marinade. Cover and refrigerate for approximately 2 hours.
Heat the grill to medium-high heat. Lightly oil the grill.
Remove the steak from the marinade and discard the marinade.
Grill the flank steak to desired degree of doneness, 3 to 4 minutes per side for medium-rare.
Let the steak rest for 5 to 10 minutes. Cut in thin slices across the grain.
Whisk together port wine vinegar (sherry vinegar will also work here), smoked paprika and salt. Slowly pour in olive oil, whisking constantly.
Putting it together:
Divide the spring greens between four dinner plates or large bowl. Top with the flank steak, red bell pepper and cucumber. Drizzle with some of the dressing and serve. Serve with additional dressing, if desired.
If you prefer, toss the salad in one big bowl and let everyone serve themselves. The steak can be grilled several hours in advance and served at room temperature.