Dip each crab in milk and them lightly coat both sides with flour. Heat butter and oil together in a large, oven proof (no plastic handles) pan over medium heat. Add crabs and sauté until crispy and golden, about 4 minutes. Place pan in preheated oven and cook 3 more minutes. Transfer crabs to paper toweling to absorb oil. Set aside and keep warm.
Make warm herb vinaigrette: In a large bowl, combine shallots, basil, extra virgin olive oil, balsamic vinegar and lemon juice. After these ingredients are incorporated, add the tomatoes and season with salt and pepper. Mix well, cover with plastic and refrigerate until ready to serve.
When ready to prepare dish, warm the vinaigrette in a sauce pan over high heat, removing from stove when it begins to simmer.
Make wilted spinach: Heat olive oil in a large skillet over medium heat. Add spinach and sauté 4 minutes, until wilted. Drain liquid.
Serve: Place wilted spinach on one side of four individual dinner plates. Arrange two soft-shell crabs over each mound of spinach, overlapping one over the other. Spoon tomato vinaigrette over crabs and around the sides of the plate.
To properly clean a soft-shelled crab: Peel back the pointed part of the shell and scrape away the gills from each side. Using sharp scissors, cut off the head and carefully squeeze out the green bubble behind the eyes. Next, bend back the apron, the thin piece that folds under the crab. Twist the apron until it breaks off, removing the intestinal vein at the same time.