PREPARE THE CREMA
In a small bowl, combine the yogurt, the sour cream, and the milk in a small bowl. Whisk to blend evenly. Cover and refrigerate.
MAKE A SPICE RUB
Heat a grill or grill pan to medium-high.
Grind the pasilla chile in a food processor (or a coffee-type grinder designated for your spices and other non-coffee items). You should have 1 tablespoon of ground chile.
In a medium bowl, combine the pasilla, the brown sugar, the canola oil, the salt, the garlic, and the jalapeño.
Lay the fish fillets on a large plate or platter and massage on all sides with the pasilla mixture. Refrigerate while you season the grill.
GET READY ... AND GRILL
Spray a clean grill rag with cooking spray and rub the grill grates liberally. Repeat until the grill is well seasoned and ready to be kind to fish.
Grill the fish fillets for about 4 minutes on each side, leaving it untouched and unturned during the first 4 minutes.
Turn the fillets once and only once, or you'll have fish bits everywhere. People will feel cheated when you try to hide massacred fillets in their tacos.
Using a thin fish spatula, lift the fish gently from the grill and place on a platter.
Now, brush the tortillas lightly with 3 tablespoons water and grill until marked and softened. This happens fast--30 seconds.
Bundle them up in a clean tea towel and lay the bundle in a bowl or basket.
Serve the fish on a platter with a large serving spoon. Present the fish alongside the tortillas, the cabbage, the cut limes, the onions, the First-Cut Salsa Fresca, the crema, and the fresh cilantro.