How I Became King by Eating Monsters

Mars, a 12-year-old prince, is terrified of being assassinated. His meals are constantly being poisoned, so he lives on monster meat. Then one night, a beautiful red-haired woman appears and says to him, "You have potential. Become my apprentice." From a single misunderstanding, the boy becomes the most powerful king (even though he doesn't want to be).

Official Japanese:
Hakusensha,
Young Animal