Preheat the oven to 400°F. Bring a large pot of salted water to a boil. Mince the garlic. Peel, halve and small dice the onion. Roughly chop the spinach.
In a medium pot, heat a drizzle of olive oil on medium-high. Add the onion and garlic and cook, stirring frequently, 2 to 3 minutes, until the onions are translucent. Add the tomatoes; season with the spice blend and S&P. Let simmer, 8 to 10 minutes, until slightly thickened.
Add the pasta shells to the pot of boiling water and cook, 10 to 12 minutes, or until al dente (still firm to the bite). Drain thoroughly and toss with a drizzle of olive oil to prevent sticking.
While the sauce and pasta cook, in a large bowl, combine the ricotta, chopped spinach, 1 egg (double for 4 portions) and ½ the asiago. Stir well to combine; season with S&P.
Spread a thin layer of tomato sauce on the bottom of an oven-safe baking dish. Working one at a time, fill each cooked pasta shell with a generous spoonful of the spinach-ricotta mixture. Lay the filled shells in the baking dish in an even layer. Spread the remaining tomato sauce over the stuffed shells. Drizzle with the pesto and top with the remaining asiago.
Bake in the oven, 12 to 15 minutes, or until the sauce is bubbly and the cheese is slightly browned. Remove from the oven and let rest for 5 minutes before serving. Divide the finished shells between your plates. Bon appétit!