how to fix the hole in the wall?

[pitufina]

In our house, the previous owner had the brilliant idea of making a doorway between the bathroom and an adjacent room. The problem is that it's a load-bearing wall, and the hole didn't turn out very well. Now we need to close up the hole, but what's the best way to do it? Masonry? Sure, you can use studs and plasterboard, but it doesn't feel sturdy enough. The hole doesn't really matter for the load-bearing wall, but it should feel like it's done properly as well.
Any tips are greatly appreciated. Attached is a picture of the mess.

 

[MathiasS]

If it is a concrete wall/masonry wall, I would fill the hole using leca-blocks. If it is a wooden wall, it's studs and drywall that apply.

 

[pitufina]

yes, the wall is a real concrete wall.

 

[Kurtivan]

Since you are not concerned about the strength, you can use metal studs and gypsum on it. Simple and quick!