Prep: Preheat oven to 400°F (200°C). Pour boiling water over spinach in a colander to wilt. Cool, squeeze dry, chop.
Tomato Sauce: Cook garlic in oil 1-2 mins. Add tomatoes, simmer, then set aside.
Cheese Sauce: Melt butter, whisk in flour, cook 2 mins. Gradually whisk in milk until thickened. Off heat, stir in Parmesan, nutmeg, salt, pepper.
Filling: Mix squeezed spinach with ricotta, lemon zest/juice, basil, ¼ cup Parmesan, salt, and pepper.
Assemble: Spread half the tomato sauce in a 9x13 dish. Pipe or spoon filling into cannelloni tubes; arrange in dish. Top with remaining tomato sauce, then cheese sauce.
Bake: Bake 25-30 mins until golden and bubbling. Rest 5-10 mins before serving.