Bring a large pot of water to a rolling boil and generously salt it (about 1 tbsp salt per 4 quarts water). The salt seasons the pasta through and improves flavor. Add the dry pasta and cook to just al dente (check the package but aim for 1–2 minutes less than 'fully done') — al dente pasta holds its shape and texture when chilled and won't go mushy in the pasta salad.

Reserve 1/4 cup of the hot pasta cooking water, then drain the pasta and immediately rinse under cold running water to stop the cooking and remove surface starch. Toss the cooled pasta with 1 teaspoon olive oil so strands/rotini don't clump while you finish the rest of the recipe. Cooling and rinsing are key to a separated, boxed-style cold pasta salad.

Whisk together the pasta-salad dressing: 2 tbsp olive oil, 2 tbsp mayonnaise, 2 tbsp lemon juice, 1 tsp Dijon, 1 clove minced garlic, 1/4 cup grated Parmesan, chopped parsley, 1/4 tsp salt and a few grinds of black pepper. The mayo gives body like a boxed mix, lemon brightens, Parmesan provides savory umami, and Dijon stabilizes the emulsion so the dressing clings to cold pasta.

Fold the cooled pasta into the dressing along with the thinly sliced red onion. Taste and adjust salt and acid — chilling dulls saltiness, so the pasta salad should be slightly brighter than you want because flavors will settle. Cover and refrigerate at least 30 minutes so the dressing melds and the Parmesan hydrates the pasta surfaces.

While the pasta chills, prepare the chicken: pat the breasts dry and lightly butterfly if very thick so thickness is uniform (this lets chicken cook evenly). Season both sides with 3/4 tsp kosher salt, 1/2 tsp cracked black pepper, 1/2 tsp smoked paprika (optional), and 1/2 tsp dried oregano. A uniform seasoning and even thickness ensure even browning and predictable internal doneness.

Heat a heavy skillet or grill pan over medium-high heat with 1 tsp olive oil until shimmering. Sear the chicken 4–6 minutes per side (depending on thickness) until a deep golden-brown crust forms — the Maillard reaction (browning) creates savory flavor and a pleasant crust. Finish to an internal temperature of 155–160°F (68–71°C); the carryover resting will bring it to 165°F (74°C). Rest the chicken 5–8 minutes before slicing to let juices redistribute, keeping meat moist.

Trim and core the peppers, then cut into 3/4" strips. In the same skillet (or under a hot broiler), cook the pepper strips in the remaining hot pan space or on a sheet pan brushed with 1 tsp olive oil. Cook over medium-high heat 6–8 minutes until edges char and skins soften — charring concentrates sweetness and adds smoky complexity. Season lightly with salt and a squeeze of lemon.

Assemble the salad: divide the baby spinach among 4 plates. Lightly dress the spinach with a small drizzle (about 1 tsp) olive oil and a few drops lemon juice — we keep the greens lightly dressed because the cold pasta salad is already dressed and rich. Mound 3/4 cup (about) of the chilled Parmesan pasta salad on one side of each plate (or serve in a chilled bowl on the side if you prefer).

Slice the rested chicken thinly against the grain and fan it over the spinach. Scatter the charred pepper strips on and around the chicken, then spoon any pan juices lightly over the meat for extra flavor. Finish with a little extra grated Parmesan and chopped parsley for color and a final bright note.

Serve immediately so the chicken is still slightly warm against the cold pasta salad. If you prefer everything cold, allow the chicken to cool and chill briefly. The contrast of warm/semi-warm proteins and cold pasta salad is a classic, restaurant-style textural play.