What can I substitute for ragu in kosher lasgne?
You can either substitute the bechamel or the ragu:

Bechamel


Use soy or almond milk and vegan butter
Ideas:


  
  http://www.onegreenplanet.org/vegan-food/how-to-make-classic-sauces-vegan/
  https://realfood.tesco.com/recipes/dairy-free-bechamel-sauce.html
  



Ragu


Simple.. just use vegetarian mince:


  
  https://www.quorn.co.uk/products/meat-free-mince
  https://www.morningstarfarms.com/products/morningstar-farms-meal-starters-grillers-recipe-crumbles-product.html
  



Both are listed as kosher. 

Additional ideas/answers already posted here: What can I substitute for dairy products in kosher meat dishes?