Cut 1 ½ pounds boneless, skinless chicken into bite-sized pieces, then season all over with 1 teaspoon of the kosher salt.

Melt 2 tablespoons unsalted butter in a Dutch oven or large pot over medium-high heat. Working in 2 batches, add the chicken in a single layer and cook until browned on both sides and cooked through, 4 to 6 minutes per batch. Transfer the chicken to a plate with a slotted spoon.

Add 1 diced medium yellow onion, 3 minced garlic cloves, ¼ teaspoon black pepper, and the remaining 1 teaspoon of the kosher salt to the pot. Reduce the heat to medium and cook, stirring occasionally and scraping the bottom of the pot, until the onion is softened and translucent, 3 to 4 minutes.

Stir in ½ cup julienne sun-dried tomatoes and 3 tablespoons tomato paste. Cook, stirring often, until the tomato paste is slightly darkened in color, 2 to 3 minutes.

Stir in 6 cups low-sodium chicken broth, 1 teaspoon dried thyme, and 1 pinch red pepper flakes if using. Bring to a boil.

Stir in 8 ounces dried medium pasta shells. Reduce the heat to maintain a simmer. Cook, stirring occasionally, until the pasta is al dente, 10 to 12 minutes.

Return the chicken and any accumulated juices to the pot. Add 5 ounces baby spinach, ¾ cup heavy cream, and ½ ounce grated Parmesan cheese. Cook, stirring often, until the spinach is wilted and the chicken is warmed through, about 2 minutes. Taste and season with more kosher salt and black pepper as needed. Serve with more grated Parmesan cheese if desired.