Preheat the oven to 375 degrees. Have ready a 13 x 9 inch oven proof dish.
Melt the butter in a large pan.
Add the flour to the melted butter and whisk over a medium heat for about 5 minutes, not stopping.
Gradually add the milk to the pan whisking well after each addition making sure the sauce stays smooth and lump free.
Once all of the milk has been added, cook for a further 5-8 minutes still whisking and keeping smooth.
Finally add the pesto and stir through. Keep the sauce warm on a very low heat while you make the enchiladas. If it gets too thick, add a few drops of milk to loosen it up again. It should be pourable like a thick soup.
In a bowl, combine the crab meat, shrimp, basil, salt, pepper and monterey jack cheese. Mix well.
Lay out the tortillas and divide the filling evenly among them.
Make sure the filling is in a line on each tortilla and start to roll them up tightly.
Pour a half cup of the sauce into the bottom of the dish and spread it out.
Place each filled tortilla into the dish until full. If there is any filling extra just stuff it between the sides of the tortilla and dish.
Pour the pesto sauce over the filled tortillas. Sprinkle the Gruyere cheese evenly over the sauce.
Bake in the oven for 30 minutes. After 30 minutes, increase the oven temperature to 400 degrees and bake for a further 10 minutes and the top is golden brown and bubbling.
Let cool for 10 minutes before serving.