Melt butter in a medium saucepan over medium heat. Add the garlic and shallots and cook until softened and the onions are beginning to caramelise and become very fragrant, about 5 minutes

Stir in the spinach, sun-dried tomatoes and diced tomatoes and cook until the spinach is wilted, about 3 minutes.

Add the heavy cream and flour and stir to incorporate.

Add the salt and pepper and bring to a boil.

Stir in the Parmesan cheese and simmer on low until the sauce thickens slightly. 5-7 minutes.

Remove from heat and stir in lemon juice

Cool slightly before constructing lasagna.