Mix the white wine vinegar, 1 tbsp olive oil, Dijon mustard, Italian herbs, garlic clove, salt, and pepper in a large bowl. Add chicken breasts and toss to coat. Marinate for at least 1 hour (or up to 24 hours in the fridge).

Heat 1 tbsp olive oil in a large oven-safe skillet or pot over medium-high heat. Add the marinated chicken breasts and sear for 4 minutes per side until golden brown and mostly cooked through. Remove chicken and set aside on a plate, cover loosely with foil.

Turn the heat up to medium-high. Add the remaining 1/2 tbsp olive oil if the pan looks dry.

Add the chopped onion and minced garlic to the skillet. Sauté for 2 minutes until the onion is softened and translucent.

Stir in the orzo pasta, diced tomatoes (with juices), chicken broth, Italian herbs, red pepper flakes, salt, and pepper. Bring the mixture to a simmer.

Reduce heat to medium-low, cover the skillet, and simmer gently for 9-10 minutes, stirring frequently to prevent sticking, until the orzo is almost al dente.

Return the chicken breasts to the skillet, nestling them into the orzo mixture. Cover and cook for another 5-7 minutes, or until the chicken is cooked through and the orzo is tender.

Remove from heat. Stir in grated Parmesan cheese and fresh basil. The residual heat will finish cooking the orzo and melt the cheese, creating a delicious, saucy consistency.