How to Make It
In a large deep skillet, bring 2 cups of water to a boil. Add the chard stems and leaves and cook over high heat until just tender, about 8 minutes. Drain, pressing on the leaves to extract excess liquid.
Wipe out the skillet. Add the olive oil and heat until shimmering. Add the garlic and cook over moderately high heat, stirring, until lightly browned. Add the tomatoes and pimentón and cook for 2 minutes. Add the cooked chard, fennel seeds and 2 tablespoons of water and cook over moderate heat for 2 to 3 minutes longer. Stir in the white wine vinegar, season with salt and pepper, transfer the Swiss chard to plates and serve.