Special equipment: a straight-sided 3-quart tall glass bowl or four 16-ounce wide-mouth glass jars with lids
Preheat an outdoor grill to medium heat. Roughly chop 2 of the tomatoes, and add to a blender, along with the lemon juice, mayonnaise, vinegar, honey, 3/4 teaspoon salt and the cayenne; blend the dressing until smooth. Poke the steak all over on both sides, and put in a resealable plastic bag. Pour in 1/2 cup of the dressing, and let the steak marinate at room temperature for 30 minutes. Lay the bread, corn and onions out on a rimmed baking sheet. Brush everything all over with 2 tablespoons of the remaining dressing, and sprinkle with 1/2 teaspoon salt. Place the baking sheet near your grill so you can transfer all the grilled ingredients onto it when they're done. Grill the steak until nicely marked on the outside and medium-rare, 7 to 9 minutes per side. Grill the onions until grill marks appear and the onions are slightly tender, 4 to 6 minutes per side. Grill the corn, turning as needed, until charred on all sides, 8 to 10 minutes. Grill the bread until nicely charred, about 2 minutes per side. Keeping the individual ingredients separate, shave the corn kernels off of the cobs, cut the onion and bread into bite-size pieces, and slice the steak against the grain into thin bite-size pieces. Chop the 2 remaining tomatoes into bite-size pieces. Using a straight-sided 3-quart tall glass bowl for 1 salad or four 16-ounce wide-mouth glass jars with lids for 4 individual salads, make a first layer with half the spinach; then add the following in separate layers: half the basil, the onion, tomato, feta, the remaining spinach and basil, the corn and bread. Top with the steak slices, and garnish with more basil leaves. (The salad can be put together, covered and refrigerated for up to 2 hours before serving.) If making 1 salad, just before serving, transfer it to a large bowl, toss with the remaining dressing and season with salt and pepper. If making 4 salads, add dressing to each jar, season with salt and pepper, cover with the lid and shake to coat.