In a bowl, dissolve the yeast in 2 tablespoons warm water. Let stand 10 minutes until frothy. Add the flour, salt, 1 cup warm water and 4 tablespoons olive oil. Knead on a floured surface until smooth, elastic and slightly tacky to the touch, about 7 to 10 minutes. Place the dough in an oiled bowl and turn it over so that it is completely coated with oil. Cover with plastic wrap and place in a warm place at approximately 75°F. Allow to rise until double in volume, 1 to 1 1/2 hours.
Preheat oven to 350°F. Pour 1 cup boiling water over all the raisins and let stand 15 minutes. Drain. Place the walnut halves on a baking sheet and toast until fragrant and lightly golden, about 7 minutes.
Remove the tough stems from the Swiss chard, wash and dry well. Warm 2 tablespoons of the olive oil in a frying pan over medium high heat. Add the garlic and stir for 5 seconds. Add the Swiss chard and raisins and cook, tossing occasionally with tongs, until wilted, about 6 to 8 minutes. Season well with salt and pepper. Place a pizza stone on the bottom shelf of the oven and increase the oven temperature to 500°F.
Punch down dough and cut into 2 pieces. Flour surface and roll one piece of the dough into a long, flat oval shape, 3/8-inch thick, 8 x 10-inches. Build up edges slightly. Place the dough on a well-floured pizza peel or paddle. Place half of the greens on the dough, distribute evenly, leaving a 1/2-inch border around the edge. Top with the walnuts and drizzle with the remaining 2 tablespoons olive oil. Bake until golden and crispy, 15 to 18 minutes.
Remove coca from pan and cut into wedges. Serve immediately or at room temperature.