After you shell the shrimp for this dish, simmer the shells in water to make a simple stock. If you wish, you can substitute large sea scallops for the shrimp. Slideshow: Amazing Shrimp Dishes
Crush the saffron threads in a small bowl. Pour in the wine and let steep while you proceed.
Heat a large nonreactive skillet over high heat. Add handfuls of the spinach, with water clinging to it, to the pan, stirring to wilt the leaves before each addition. Continue until all the spinach has wilted. Transfer the spinach to a colander to drain well and then chop it coarsely. Wipe out the skillet.
Melt the butter in the skillet over moderate heat. When the foam subsides, add the shrimp, season with salt and pepper and cook, stirring, until pink and starting to curl, about 3 minutes. Transfer the shrimp and any cooking juices to a plate.
Heat the oil in the skillet. Add the shallots and cook over moderate heat until softened, about 4 minutes. Add the garlic and cook until fragrant, about 1 minute. Increase the heat to moderately high and add the wine with the saffron. Boil until the wine is reduced by half.
Stir in the heavy cream and shrimp stock and bring to a simmer. Add the spinach and tomato and simmer for 1 minute. Add the shrimp with their juices and cook until just heated through. Season with the lemon juice, salt and pepper and serve at once.