Preheat oven to 350°F (175°C).

Grease a 1 1/2-quart shallow baking dish.

In a medium saucepan, cook chopped onion in butter until transparent.

Add condensed cream of mushroom soup, sour cream, milk, lemon juice, seasoned salt, thyme, dry mustard, and pepper to the saucepan and mix well.

Stir in cooked and drained noodles and defrosted, drained spinach.

Place half of the noodle mixture into the prepared baking dish.

Cover with flaked tuna and half of the shredded Swiss cheese.

Repeat with the remaining noodle mixture and cheese.

Sprinkle with paprika.

Bake for 20 to 25 minutes, or until the edges are bubbly and the cheese is melted and lightly browned.

Let stand for 5 minutes before serving.