Preheat the oven to 350°F. Spray a 9 x 13-inch baking dish with cooking spray.

Heat a small skillet over medium-high heat. Add the oil and swirl to coat. Add the onion and sauté until tender, 4 to 5 minutes. Set aside to cool.

In a food processor, pulse the pecans and flour until the pecans are finely chopped. Transfer to a bowl and stir in the melted butter until a dough forms.

Press the dough evenly into the bottom of the baking dish. Bake until browned and toasted, 25 to 28 minutes. Set aside to cool for at least 5 minutes. Leave the oven on.

In a large bowl, whisk together the sour cream, heavy cream, dill, salt, pepper, and eggs. Stir in the sautéed onion and shredded chicken and pour evenly over the crust mixture.

With a long side of the baking dish facing you, start at the left short side and lay the asparagus evenly across the width of the chicken mixture and parallel to one another. Sprinkle evenly with the Cheddar.

Bake until the cheese is slightly browned and bubbling on the edges, about 35 minutes. Let cool for at least 30 minutes before serving. Cut into squares. Store in an airtight container in the refrigerator for up to 3 days.