Put the tomatoes, garlic, onion, and jalapeno in a shallow baking dish and place on the grill grate before starting your Traeger. (This will expose the vegetables to more wood smoke.)
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 450F and preheat, lid closed, for 10 to 15 minutes.
When the grill is hot, arrange the New Mexican chiles and the sweet corn on the grate and grill until the chiles are blistered and blackened in spots and the corn is lightly browned, 15 to 20 minutes for the chiles and 10 to 15 minutes for the corn, turning with tongs as needed.
Stir the tomato-onion mixture once or twice and remove it from the grill grate when the tomatoes begin to break down. Let all the vegetables cool.
Reduce the heat of the Traeger to 350F if you intend to bake the rellenos right away. (You can also make the sauce and assemble the rellenos the day before you bake them. Cover and refrigerate.)
Put the cooled tomato mixture in a blender and liquefy. Pour into a saucepan.
Stir in the cumin, oregano, chile powder, cinnamon, and salt and pepper to taste. Simmer over medium heat for 15 to 20 minutes, or until the sauce is slightly thickened, stirring occasionally.
Carefully peel the blistered outer skin off the New Mexican chiles: Leave the stem ends intact, and try not to tear the flesh.
With a small paring knife, slit each chile lengthwise from the shoulder (just below the stem) to the tip. Pull out the seeds and set the chiles aside while you make the filling.
Slice the corn off the cobs and put in a large mixing bowl. Toss with 2 cups of the cheese, reserving 1 cup. Gently stir in the sour cream. Season with salt and pepper. Generously stuff the chiles with the corn-cheese mixture and arrange shoulder to shoulder, cut sides up, in a baking dish or on a rimmed baking sheet. (Line with foil for easy clean-up, if desired.) Sprinkle some of the reserved cheese on top of each relleno.
Bake the rellenos for 25 to 30 minutes, or until the filling is bubbling and the cheese has melted. Reheat the tomato sauce if necessary.
To serve, put a small pool of tomato sauce on each plate and arrange a relleno in the center of it. Sprinkle with queso fresco and garnish with fresh cilantro leaves, if desired. Serve immediately.