Preheat an oven to 375.
Place a medium to large saucepan over medium-high heat and drizzle in the oil. Add the onions and saute lightly for 5-10 minutes until they are tender and just barely picking up some golden color.
While they cook, squeeze the greens dry. Using your hands or a dish towel, ball up the greens and squeeze well until very little liquid is coming out. Place in a bowl with the minced garlic and ricotta cheese. Fold together.
Move the onions to the outside edges of the pot and add the ground beef to the center. As soon as the meat is broken up and cooked through, add in the crushed tomatoes (reserve about 1/2 cup of the tomatoes), Italian seasonings and cooked onions. Bring up to a simmer for a few minutes until the sauce is melding together. Season with a bit of salt and pepper to taste.
To layer the lasagna, start by placing the reserved tomatoes in the bottom of a small-medium size casserole pan (about 8×8, 9×9 or equivalent, unless recipe is doubled). Lay the dry noodles on top of the tomatoes. Dollop half of the ricotta-spinach on top, spreading out along the dry noodles.
Add more pasta, meat sauce, pasta, ricotta-spinach, pasta, sauce and then finish with the shredded mozzarella.
Cover with foil and place in the oven for 45 minutes. Remove the foil and bake for 5-15 more minutes until the top is golden and a knife easily cuts through the center.
Remove from the oven and allow to rest for at least 5 minutes for everything to set. This rest time will make it less runny when you cut into it.