Cook the jumbo shells in a large pot of boiling, salted water according to package directions until al dente.
Drain well in a colander, rinse with cold water to stop the cooking process, and set aside.
While the pasta cooks, heat the olive oil in a large skillet over medium-high heat.
Add the shredded carrots, zucchini, and chopped onion.
Sauté for 3 to 5 minutes, until the vegetables are tender-crisp.
Stir in the thawed and squeezed-dry spinach and cook for one more minute.
Transfer the vegetable mixture to a large bowl.
To the bowl with the cooked vegetables, add the ricotta cheese, 1 cup or 113g (4 oz) of the Italian cheese blend, salt, and cayenne pepper.
Stir until everything is well combined. Carefully spoon about 2 heaping tablespoons of the cheese and vegetable mixture into each cooked pasta shell.
Pour the pasta sauce into the same skillet you used for the vegetables and spread it evenly. Gently nestle the filled shells into the sauce, opening-side up.
Bring the sauce to a simmer over medium heat.
Cover the skillet and cook for about 10 minutes, or until the shells are heated through and the sauce is bubbly. Uncover, sprinkle the remaining ½ cup or 57g (2.01 oz) of Italian cheese over the top, and serve immediately.
Squeeze the thawed spinach as dry as possible. Excess water will make the filling runny. A potato ricer or wrapping the spinach in a clean kitchen towel and wringing it out works wonders.
