Preheat the oven to 400 degrees. Position the racks to the center and lower third of the oven.
Scoop the potatoes and onions into a large bowl and toss with 1 1/2 tablespoons of olive oil and 1 teaspoon salt. Spread in an even layer onto a rimmed baking sheet and slide onto the lower third rack. Set the timer for 30 minutes. Using the same bowl, combine the chicken and carrots with the remaining oil and 1 teaspoon salt. Spread in an even layer on another rimmed baking sheet and after 10 minutes have passed on the timer, place this baking sheet on the center rack.
When the timer goes off, remove both baking sheets. Scrape the potatoes and onions into a large bowl. Using a slotted spoon, transfer the chicken and carrots to the bowl. Pour the tomatillo salsa over the top along with the crema or heavy cream and the serranos. Toss until everything is well covered by the salsa/crema mixture. Divide equally between 4 cazuelas, sprinkle cheese over the top of each one and slide onto the center rack. Bake for 10 to 15 minutes or until the cazuelas are hot and the cheese is melted.
Serve immediately with a sprinkling of cilantro over the top.