Lay each chicken breast flat on a cutting board and slice it horizontally into two thinner cutlets. Thinner pieces cook faster and more evenly than one thick breast, which is the biggest reason meal prep chicken sometimes turns out dry. Toss the cutlets with olive oil, oregano, paprika, onion powder, salt, and pepper, then let them sit for a few minutes so the seasoning soaks in.
Heat a skillet over medium-high heat and add the seasoned chicken once it's hot. Cook for about 5 minutes per side without moving the pieces around, so a golden crust has time to form. The chicken is ready when it hits 165°F in the thickest part. Rest it on a plate for a few minutes before slicing so the juices redistribute.
While the chicken rests, bring a couple cups of water to a boil in a large pot with a steamer basket set inside. Add the cauliflower florets, cover, and steam for 6 to 8 minutes until they're completely fork-tender. Let the cauliflower sit uncovered for a few minutes afterward so excess steam can escape before blending.
Transfer the steamed cauliflower and garlic to a food processor and blend until smooth. Stir in the butter, freshly grated Parmesan, salt, and pepper by hand. Taste and adjust the seasoning until the mash feels rich and creamy rather than watery or bland.
Using the same skillet the chicken cooked in, add the remaining olive oil and butter over medium heat. Toss in the red onion, bell pepper, and zucchini, and sauté for 4 to 5 minutes until just tender. Season with garlic powder, salt, and pepper, keeping a little bite in the vegetables so they hold up through the week.
Slice the rested chicken and divide it evenly among four containers along with the mash and sautéed vegetables. Let everything cool completely before sealing the lids, since trapped heat creates condensation that makes the food soggy. Refrigerate for up to four days and reheat individual portions as needed.