1bunchwatercresswashed, dried thoroughly, and trimmed of tough stems
1bunchflat-leaf parsleywashed and dried thoroughly
Make the vinaigrette
In a small bowl, whisk together the lemon juice, cilantro, garlic, salt, cumin, paprika, and cayenne. Add the oil in a slow, steady stream, whisking constantly until smooth. Season with black pepper to taste.
Prepare the grill
If using a gas grill, preheat it on medium to medium-high. If using a charcoal grill, light a large chimney starter filled with hardwood charcoal (about 6 quarts) and allow to burn until all the charcoal is covered with a layer of fine gray ash. Build a single-level fire by spreading the coals evenly over the bottom of the grill. Set the cooking grate in place, cover the grill with the lid, and let the grate heat up, about 5 minutes. Use a grill brush to scrape the cooking grate clean. The grill is ready when the coals are medium-hot.
Cook the tuna steaks
Cut the tuna steaks in half to make 4 equal pieces. Brush the tuna with the olive oil and sprinkle generously with salt and pepper.
Lightly dip a small wad of paper towels in vegetable oil. Holding the wad with long-handled tongs, wipe the cooking grate. Grill the tuna, uncovered and turning once using a metal spatula, and then continue to cook until the desired doneness, 4 to 5 minutes for rare or 6 to 7 minutes for medium-rare.
While the tuna is on the grill, place the watercress and parsley in a medium bowl. Drizzle half the vinaigrette over the greens and toss well. Divvy the greens among 4 individual plates.
Place one grilled tuna steak on each plate over the salad greens. Drizzle the remaining vinaigrette over the steaks and serve immediately.