Depends on your lasagna recipe. In any case surely its up to us to use our common sense and adapt our recipes with an eye to healthy eating.
Yesterday, for breakfast I had fruit, cereal and semi-skimmed milk. for lunch I had a huge bowl of home-made veg soup with, IIRC, 7 vegetables in it. For dinner I had lasagna as it happens - two layers of lasagna sheets, with tomatoes, celery, carrots, garlic, onions, mushrooms, ground beef, chicken livers wine, home made beef stock, sauce with two sorts of cheese, bechamel sauce (semi-skimmed milk) & seasonings (my Italian neighbour's recipe). All in one pot. Didn't need or want, salad, bread or anything else with it. It all fitted in to the Slimming World plan, well within my "syns" & "healthy extras" allowances and I don't think any of it was unbalanced.
By the way, I've lost 12 pounds in the last few weeks on SW