Heat the olive oil and butter in a large pot over medium heat. Add the chopped onion, a pinch of salt, and the garlic. Cook for 3-4 minutes until the onion is soft and fragrant.
Add the bacon, carrot, celery, and zucchini. Sauté for about 5 minutes until the vegetables begin to soften.
Stir in the risoni, then pour in the chicken stock. Add the parmesan rind and the chicken. Increase the heat to high, cover with a lid, and bring to a simmer. Cook, stirring occasionally, until the risoni is tender and the chicken is cooked through this should take around 10–12 minutes.
Remove the parmesan rind and take out the chicken. Shred the chicken using two forks, then return it to the pot.
Stir in the lemon zest, lemon juice, freshly grated parmesan, and spinach. Cook for a further 1-2 minutes, until the spinach has wilted.