Heat oil in large skillet over high heat. Add onion; saute until soft, about 8 minutes. Place in medium-sized bowl.

Add tomatoes to skillet, breaking up with spoon. Add garlic and carrots. Bring to boiling. Lower heat; simmer, covered, 10 minutes. Uncover and simmer 10 minutes. Stir in basil and salt. Set sauce aside.

Stem and wash fresh spinach. Place in large pot over high heat; cook until wilted, about 3 minutes. Drain and squeeze dry.

Cook lasagna noodles according to package directions. Drain.

Heat oven to moderate (350'F.). Line 9" springform pan with aluminum foil; grease foil.

Add ricotta, Parmesan and pepper to reserved onion; stir to combine.

Trim lasagna noodles to fit pan. Fit 4 noodles into pan, overlapping slightly. Cover with one-third ricotta mixture, spreading evenly. Top with one-third spinach, one-quarter sauce and one-quarter fontina.

Repeat two more times, using noodle trimmings as one layer. Top with the last of the noodles, the remaining quarter of sauce and fontina.

Bake until hot, 50 minutes to an hour. Let stand 5 minutes before serving. Remove sides of pan.