In a large salad bowl, toss the spinach and basil. Cut each slice of bacon into thirds. Cook in a medium skillet over medium to medium-high heat, stirring occasionally, until crisp; drain on paper towels. Reserve 1 Tbs. of the bacon fat. Crumble the bacon into pieces.
In a medium metal bowl, whisk the vinegar with the mustard. Slowly whisk in 1/3 cup of the oil and then whisk in the 1 Tbs. bacon fat. Season to taste with sea salt and pepper. Set aside in a warm place.
Position an oven rack as close as possible to the broiler element. Put a heavy-duty rimmed baking sheet on the rack and heat the broiler to high. Toss the tomatoes with the remaining 2 tsp. olive oil, the herbes de Provence, dehydrated garlic, and kosher salt. Pour the tomatoes onto the hot pan and broil, stirring occasionally, until the skins are cracked and blistered and the flesh is warmed through, 4 to 5 minutes. Turn off the broiler. Transfer the tomatoes with a slotted spoon to the bowl with the salad dressing. Stir to coat and mix the seasonings into the dressing. Put the bowl in the oven to keep warm until ready to serve.
Just before serving, transfer the tomatoes from the dressing to another bowl with a slotted spoon. Whisk the dressing to recombine. Add half of the bacon to the greens. Drizzle the greens with 3 Tbs. of the dressing and toss lightly to coat. Add more dressing only if needed; don’t overdress the salad. Mound the greens on four salad plates. Garnish with the tomatoes, the remaining bacon, and some of the candied walnuts (you’ll have leftovers for snacking). Serve immediately.