1) To prepare the chillis: Position a rack as close as possible to the grill element and preheat. Put the chillis on a foil-lined grill pan and cook, turning occasionally with tongs, until the skin is charred, about 10 minutes. Transfer the chillis to a bowl, cover, and cool for 10 minutes.
2) Carefully rub the charred skin off the chillis. Using a small knife, make a lengthwise slit along the side of each chilli to form a pocket. Carefully cut out and discard the seeds.
3) Cut the cheese into 6 (1/2-cm-thick) slabs, about 3/4 the length of each chilli (your chillis probably vary in length, so tailor the cheese to the chillis). Slip the cheese pieces into the pocket of each chilli so they're 2/3 full. (If the cheese protrudes, just cut a little off.) "Sew" each chilli shut with a wooden skewer or long toothpick. (The skewers should be longer than the chillis, so they can be easily pulled out after frying.)
4) Whisk the flour, baking powder, cumin, and salt in a large bowl. Stir in the beer to a make a smooth batter.
5) In a large, wide, heavy-bottomed pan, pour in the oil to a depth of about 7-cm. Heat over medium heat until a deep-fry thermometer inserted in the oil registers 185C.
6) Put the flour for dredging on a plate. Working in 2 batches, dredge the chillis in the flour (the dampness of the chillis creates a light paste with the flour that seals over any tears), dip in the batter, and carefully add to the oil. Fry, turning the chillis once, until golden brown and crispy, about 4 minutes per batch. Using tongs, transfer the chillis rellenos to a dry kitchen towel-lined baking sheet to drain. Sprinkle with salt, to taste. Gently pull out and discard the skewers.
7) Heat the sauce. Spoon some of the sauce on a plate and set 1 chile relleno on top. Repeat with the remaining sauce and chiles. Serve.
8) For the tomato sauce: In a blender, combine all the ingredients and puree until smooth. Transfer the tomato puree to a medium saucepan and bring to a boil. Lower the heat and simmer until slightly thick, about 10 minutes.