Bring a pot of salted water to a boil and cook the spaghetti according to package directions until al dente, reserving 0.25 cup of pasta water before draining.
While pasta cooks, dice the chicken breast into bite-sized pieces and season with sea salt and black pepper.
In a large skillet over medium heat, add the diced pancetta and cook until crispy, then remove with a slotted spoon and set aside on a paper towel.
Add the olive oil to the same skillet and sear the chicken pieces until golden brown and cooked through, about 5-6 minutes.
Lower the heat and add the minced garlic, sautéing for 1 minute until fragrant.
Whisk together the Greek yogurt, grated Parmesan, and lemon juice in a small bowl, then stir it into the skillet along with the reserved pasta water to create a smooth sauce.
Add the cooked spaghetti and baby spinach to the skillet, tossing constantly until the spinach is wilted and the sauce coats the noodles.
Top with the crispy pancetta and an extra crack of black pepper before serving.