Shimano M355 part broken
I know what that is. It’s the pivot for the brake lever, and Shimano does not sell extras. Your options are:

Buy a new brake system or lever (depending on which is cheaper. Used is fine as well. All Shimano MTB brakes are inter-compatible.)
Go to your LBS and ask if they have a lever you can cannibalize. A friend of mine broke the exact same part and our LBS happened to have a spare lever they keep around for spare parts.

Be careful while fiddling with that 2mm hex fitting. It’s easy to strip out, and fixing that will definitely mean a new lever.