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FILL a medium-sized pan with water and salt and bring to a boil. REMOVE the ends of the asparagus and divide them into halves. COOK the asparagus for a few minutes in the boiling water, transfer them to a bowl and bring the water back to a boil.
COOK the spinach shortly if needed and transfer to a fine sieve to remove as much water as possible. GRATE the peel and squeeze the juice form the lemons.
HEAT 50 gram dairy butter in a saucepan and ADD the flour all at once. Gradually ADD the milk, make sure to whisk well to create a smooth sauce. You'll notice the bechamel sauce will thicken very quickly.
ADD the lemon juice and zest and SEASON with pepper and salt. PRE-HEAT the oven to 180 degrees Celsius or 350 degrees Fahrenheit and coat the oven dish with butter.
DIVIDE the sausages into smaller pieces, but not too small. HEAT a bit of butter in a skillet and cook the sausages for a few minutes. ADD the peas and season with pepper and salt.
DIVIDE a layer of spinach on the bottom of the oven dish and then about 4 lasagna sheets. DIVIDE a layer of lemon-bechamel sauce and about 1/3rd of the sausage-peas mixture and about 1/3rd of the asparagus.
REPEAT until all the ingredients have been used and finish with a layer of lemon-bechamel sauce and a layer of grated cheese. BAKE the lasagna for about 40 minutes, or until the cheese is beautiful golden brown.
The exact cooking time depends on your oven, enjoy!