I just had this for lunch and it was very filling, tasty, quick and cheap.
It is inspired from an old recipe my sister made which I will also give you for your other "non-low carb" family members:

My new recipe, recommended for OWL or maintenance only:

I guess I'll name it "TORTUNA":

1 (7 inch) low carb tortilla (Mission brand) - 7 grams of net carbs
1 (6 oz.) can of Albacore tuna, drained - 0 carbs
1 or 2 tablespoons of cream of mushroom soup, undiluted (or your choice of cream soups), just enough to hold the tuna together and give a little flavor) - about 2 grams of carbs per tablespoon
1 or 2 tablespoons minced onion - 1 carb?
2 slices American cheese - about 1 carb per slice ?

Lightly toast tortilla on both sides in skillet with a little oil.

Mix the drained tuna, soup, and onion and spread all of it onto the toasted tortilla. Top with cheese, broken in pieces to place uniformly over the top. Place in broiler and broil until cheese is bubbly or starts to brown.

I would guess the total carb count for this is 12 to 14.

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My sister's old recipe (not low-carb)

Open-Face Toasted Tuna Sandwiches:

1 6 oz. can of tuna, drained
1 can of cream of mushroom soup, undiluted
2 tablespoons minced onion

Mix and spread 2 or 3 tablespoons of mixture onto a half of a hamburger bun. Top each with 1/2 slice American cheese (broken into half diagonally). Place under broiler until sandwich is hot and cheese starts to melt and get bubbly.