In a large heavy saucepan, heat the olive oil over medium heat. Add the onions and saute, stirring occasionally, for 5 to 6 minutes, or until softened. Add the garlic and cook for 2 minutes more, until its aroma is released.
Add the chicken stock, water, and salt to the pan and increase the heat to medium high. When the liquid is simmering, gradually sprinkle the polenta over in a very slow, thin stream, whisking constantly in the same direction until all the grains are incorporated and no lumps remain. Reduce the heat to very low. Switch to a wooden paddle and stir thoroughly every few minutes for 25 to 30 minutes, or until the mixture pulls away from the sides of the pan and the grains of polenta have softened. Stir in the butter, thyme, pepper, and grated Manchego and remove from the heat. Spoon some polenta into each of 6 warmed shallow soup bowls and top with some of the roasted peppers and chopped cilantro.
Note: Fresh chiles and bell peppers can be roasted over a gas flame or on a tray under the broiler. Keep turning so the skin is evenly charred, without burning and drying out the flesh. Transfer the charred peppers to a plastic bag, tie the top closed and let steam until cool to the touch, about 15 minutes. The best way to peel is just to pull off the charred skin by hand and then dip the peppers briefly in water to remove any blackened bits. Do not peel the pepper under running water since that will wash away flavorful juices. Once peeled, cut away stems, seeds and veins.