Butterfly the chicken breasts, cover with plastic wrap, and gently pound to flatten, taking care not to rip any holes in the meat.
Season the breasts with salt and pepper to taste.
Lay 2 slices of cheese across each breast, then layer slices of pepperoni on top of the cheese.
Tightly roll the chicken into packages, starting with the narrower end, and make sure there is no filling visible.
Place the breasts seam side down on a cookie sheet and freeze for around 2 hours.
Whisk together the egg and milk, then set up a breading station in three bowls, with flour in the first, egg wash in the second, and breadcrumbs in the final.
Remove the chicken from the freezer and bread by dredging the breast in the flour, patting off any excess, then dip in egg wash, and finally place into the seasoned breadcrumbs, pressing the crumbs into the meat to make sure they adhere.
When the chicken is golden brown, remove to a foil lined cookie sheet with a rack.
Place the chicken in the oven and cook until an internal temperature of 170°F is reached, approximately 20 minutes.
While the chicken is cooking, sauté́ the onions over medium high heat, and when translucent, add the spinach.
Deglaze the pan with the white wine, bring to a boil, add the heavy cream, and simmer for about 5 minutes.
Take the spinach off the heat and add the tarragon leaves.
Add the mixture to the blender and puree, carefully, until smooth.
Strain through a fine sieve, season with salt and pepper to taste, and reserve.
When the chicken is done, remove from the oven and allow to rest for 5 minutes.
Slice the chicken breast and serve on a generous pool of the spinach mixture.
Serve with a simple salad or side of pasta.