Preheat the oven to 375°F (190° C). line a baking sheet with aluminum foil. 

Arrange the chicken on a large plate. Brush on both sides with the canola oil and season all over with the cumin, chili powder, and salt. Arrange on the prepared pan and bake until opaque throughout, about 25 minutes. Remove from the oven and let cool. When cool enough to handle, using your hands or 2 forks, shred the meat into bite-sized pieces.

Preheat the broiler.

Spread the tortilla chips in a single layer in a large cast-iron pan or on a clean baking sheet. Top with the black beans, then the chicken, and then the Monterey jack. Slip under the broiler and broil just until the Monterey jack is melted, about 3 minutes.

Remove the nachos from the broiler and, using a large, wide spatula, carefully transfer to a serving platter. Garnish with the tomatoes, avocado, and pickled jalapeños, if using. Dollop with the sour cream and serve right away.