Cook noodles in salted water according to package directions, ensuring to not overcook (even undercooking by a minute is good as the noodles cook further when the casserole is in the oven).

Melt the butter in a medium saucepan. Add onions cook on medium heat until soft, about 5 minutes. Add in kale and stir for another minute. Add the flour and a pinch of salt and stir well, cooking an additional 2-3 minutes on medium-low heat

Preheat oven to 375° and grease a casserole dish (I used a 12 x 8 dish)

Slowly whisk in the broth until well combined, increasing heat to medium and whisking well for 30 seconds, then add the milk and bring to a boil. Reduce heat to medium and simmer, stirring occasionally until it thickens (about 7 to 9 minutes). Add drained tuna, stirring another minute.

Remove from heat and add the noodles to the sauce and mix well until evenly coated. Pour into the buttered casserole dish and top with the cheeses and flaxseed blend. Bake for 25-30 minutes. Remove from the oven and turn on the broiler. Place under the broiler a few minutes to brown the top, ensuring to not burn