How to Make It
Heat oil in a large skillet over medium-high. Sprinkle chicken with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add chicken to skillet; cook, without stirring, until chicken begins to brown, about 4 minutes. Add tomatoes, onion, garlic, and rosemary. Cook, stirring occasionally, until onion is tender and tomatoes begin to soften, about 3 minutes.
Whisk together stock and flour in a bowl until combined. Add to chicken mixture; stir and scrape browned bits from bottom of skillet. Bring to a boil. Cook, stirring often, until sauce thickens, 3 to 4 minutes.
Heat rice according to package directions. Place hot rice in a medium bowl; add spinach, lemon zest, lemon juice, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon pepper. Toss to coat (hot rice will wilt the spinach). Divide rice mixture among 4 plates; top evenly with chicken mixture.