The recipe can easily be doubled to serve 6 as a main course. Slideshow: More Salmon Recipes
Cut 6 tablespoons of the butter into 1/2-inch dice and chill. In a medium saucepan, melt 2 tablespoons of the remaining butter. Add the onion, shiitake stems, tomato, sliced garlic, peppercorns and bay leaf and cook over moderately low heat until the vegetables are soft but not browned, about 12 minutes. Add the balsamic vinegar and water and cook over moderately high heat, stirring occasionally, until the liquid is almost syrupy, about 4 minutes.
Reduce the heat to low and whisk in the 6 tablespoons of chilled diced butter, 2 to 3 pieces at a time. When all the butter has been added, season the sauce with salt and pepper. Strain the sauce through a very fine sieve into a saucepan and keep warm; discard the solids.
Spear the whole garlic clove securely with a dinner fork. In a large skillet, heat 2 tablespoons of the olive oil over high heat until just beginning to smoke. Very carefully add the spinach and stir with the garlic clove-tipped fork until wilted. Season with salt and pepper and drain.
Wipe the skillet clean and add 3 tablespoons of the butter. Add the shiitake caps and cook over moderate heat, stirring, until softened, about 3 minutes. Stir in the corn and cook until tender, about 3 minutes. Season with salt and pepper and transfer to a bowl; keep warm.
In the same skillet, melt the remaining 1 tablespoon of butter in the remaining 1 tablespoon of oil over high heat. Season the salmon with salt and pepper and sear the pieces in the hot pan, turning once, until browned but barely cooked through, about 3 minutes per side.
Mound the spinach in the center of 6 plates and surround with the corn and shiitakes. Set the salmon on the spinach and spoon the balsamic sauce on top. Garnish with the chives and serve.