Start the crème fraîchè a few days before you plan to make the salad.
- Make the crème fraîchè: In a bowl, stir together cream and buttermilk. Cover and set aside at room temperature until thick, at least 24 hours. Once thickened, stir together with remaining ingredients and refrigerate until chilled.
- Make the pickled corn: In a small, nonreactive pot, combine jalapeño, bay leaf, cider vinegar, sugar, and salt with ½ cup water. Bring to a boil, then remove from heat and add corn. Set aside to cool, then transfer to a glass jar and refrigerate until ready to use.
- Make the cornbread croutons: Place an 8- or 9-inch cast iron skillet in oven and preheat to 425 degrees. In a mixing bowl, whisk together cornmeal, salt, pepper, baking powder, and baking soda. In another bowl, whisk together buttermilk and egg. Once oven has preheated, add egg mixture and 4 tablespoons melted fat to cornmeal mixture and stir with a rubber spatula until just combined.
- Carefully remove hot skillet from oven and add remaining tablespoon of fat, tilting skillet to coat. Quickly transfer batter to pan and return to the oven. Bake until edges are brown and center is set, about 20 to 25 minutes. Set aside to cool. Once cornbread is cool to the touch, cut about ¼ of it into slabs and set aside. Reserve the rest for another use.
- Char the vegetables: Preheat a grill for medium heat. (If using a charcoal grill, prepare for two-zone cooking.) Cut squash in half lengthwise and scoop out seeds, then slice crosswise into ¼-inch-thick rounds. Toss squash, onion, okra, and corn in enough oil to coat. Sprinkle with salt. Arrange vegetables on grill and cook, turning every minute or so, until outsides are beginning to char and become tender, about 6 to 10 minutes. Set aside half of the onion; transfer remaining vegetables to a large mixing bowl. Cut corn off the cob and toss kernels with other vegetables. Set aside.
- Make the burnt onion vinaigrette: In a blender, combine vinegar, mustard, olive oil, salt, and pepper. Add reserved charred onion and blend until mixture is smooth. Adjust seasoning to taste.
- Assemble the salad: Lay cornbread slabs on hot grill to toast (or heat a cast-iron skillet on the grill and toast cornbread with more bacon fat or butter). Cut toasted cornbread into pieces.
- Spread ranch crème fraîchè on a serving platter or four individual plates and top with a few leaves of lettuce. Toss together charred vegetables, pickled corn, and enough vinaigrette to lightly coat. Arrange mixture on top of lettuce. Top with cornbread croutons and garnish with cherry tomatoes.