Preheat the oven to 425 degrees F.
On a rimmed baking sheet, add the tater tots and spread them out into an even, single layer.
Bake the tots until they are browned and crispy, about 25-30 minutes.
In a small bowl, add 1/2 of the red onions and 2 tablespoons of the pickle juice.
Let the onion mixture sit, while tossing the onions occasionally, until ready to use.
In a large, cast-iron skillet over medium heat, add the olive oil.
Add the remaining red onions to the hot oil and cook, while stirring occasionally, until tender and golden, about 6-7 minutes.
Add the beef and the steak seasoning to the hot oil and cook, while crumbling the beef, until it is no longer pink, about 5-7 minutes.
Add the tomato paste to the beef mixture and cook, while stirring continuously, until it is caramelized and darkened, about 2-3 minutes.
Sprinkle the beef mixture with the flour and cook, while stirring, until the raw flour smell disappears, about 1 minute.
In a small bowl, add the beef bouillon base to the warm water and stir to combine until the bouillon is completely dissolved.
Gradually add the bouillon mixture to the beef mixture while stirring gently.
Bring the beef mixture to a simmer and cook, while stirring occasionally, until it is slightly thickened, about 3-5 minutes.
Transfer the skillet from the heat.
Sprinkle the beef mixture with 1 cup of the cheese.
Top the cheese layer with the tots.
Sprinkle the remaining cheese over top of the tots.
Bake until the cheese is melted and the filling is bubbling, about 5-6 minutes.
In a small bowl, add the mayonnaise, the ketchup, the mustard, and the remaining pickle juice and whisk to combine.
Drain the pickled onions, discarding the pickle juice.
Scatter the pickled onions over the cheese layer in the skillet.
Sprinkle the tomatoes and the pickles over the top of the cheese-pickled onion layer.
Serve with the sauce mixture on the side.