1
Combine the lemon, parsley, garlic, oil and salt in a blender until mostly smooth.
2
Toss the chicken with the gremolata and marinate overnight or for at least 4 hours.
3
Preheat the oven to 350°F.
4
Remove the chicken from the marinade, discard marinade and season chicken with salt and pepper. Roast on a baking sheet until cooked through, about 22 minutes.
5
In a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth.
6
For the tomatoes, in a small bow, combine the parsley, breadcrumbs, parmesan, salt and pepper.
7
Slice the tomatoes in half lengthwise and remove the seeds. Arrange on a parchment paper-lined baking sheet and sprinkle with salt and pepper. Divide the filling between the tomatoes and roast for 12-15 minutes, until crisp and the cheese starts to brown slightly.
8
For the pesto, in a food processor, blend the basil, arugula, garlic, parmesan, olive oil, pine nuts, lemon juice, and salt and pepper until smooth. Adjust seasoning.
9
To serve, divide the mashed potatoes between plates. Top with chicken, roasted tomatoes, and pesto.