Warm a large pot of olive oil to 200 degrees F. Season fish with salt and pepper. Submerge in oil. Add tomatoes and submerge. Cook slowly until fish is done, about 15 minutes. Remove fish.
In a saute pan over medium-high heat, cook the lardons until crispy. Remove and pour out most oil. Add garlic and cook until slightly brown. Add spinach and cook until wilted. Remove and keep warm. Add fish and turn, to quickly crisp skin on all sides. Serve fish with tomatoes atop spinach and sprinkle with lardons.