1. In a large bowl, whisk together the ricotta, Pecorino Romano, pesto, half the mozzarella, and the salt and pepper.
2. In the bottom of a large, deep skillet w/a lid, spread one third of the tomato sauce in an even layer. Arrange 3 of the noodles over the sauce, breaking or cutting them up if necessary to make one layer. Drop half the cheese mixture, by spoonfuls, over the noodles and spread w/a rubber spatula to make an even layer. Spread half the spinach leaves over the cheese.
3. Repeat the layers w/another third of the sauce, the remaining 3 noodles, the remaining ricotta mixture, and the remaining spinach. Top w/the last third of the sauce, and sprinkle w/the remaining half of the mozzarella.
4. Place the skillet on the stove and bring to a boil over high heat. Cover, reduce the heat to medium-low, and cook, regulating the heat if necessary so that the sauce is bubbling gently around the edges the whole time, until the noodles are tender and the cheese on top is melted, about 25 min. (The lasagna can be cooked up to 1 hr. ahead and kept at cool room temp. Reheat for a few min. over low heat.)
5. Let the lasagna stand for 5 min. before cutting into wedges to serve.
