Preheat oven to 425 degrees F.
In a large bowl, combine the potatoes, carrots, onion, bell pepper, oil, garlic, 1 teaspoon salt, rosemary, and ½ teaspoon pepper and toss to coat. Transfer the veggies to a large rimmed baking sheet and spread them out in an even layer. Set aside.
In a small bowl, mix the Italian seasoning, smoked paprika, remaining ½ teaspoon salt, and ¼ teaspoon pepper. Season the chicken with the spice mixture and set the pieces over or in between the veggies (it’s okay if some of the veggies are under the chicken). Roast until a thermometer inserted in chicken reads at least 170 degrees F and the vegetables are just fork tender, about 45 to 50 minutes.
Remove the baking sheet from the oven and transfer the chicken to a plate and tent with foil to keep warm. Scatter the spinach over the vegetables and toss with a spatula to mix it with the roasted veggies and any cooking liquid. Return the baking sheet to the oven for an additional 5 to 8 minutes, or until the vegetables are tender and the spinach is wilted. Remove the baking sheet from the oven and toss the vegetables and spinach with a spatula.
Return the chicken to the sheet pan and squeeze the juice from half a lemon over the top of everything.
Slice the remaining half a lemon into wedges. Serve the chicken with the veggies with the extra lemon wedges on the side.