Infer generic types based on expression

I am building a DSL in C# using expression trees and one of the challenges I am trying to solve is inferring generic types from an input expression. Here is a contrived example:

class GenericThing<T> {
    public List<IOption<T>> solveMe;
}

// given the type of some expression, (and assuming types are assignable to each other)
var someExpression = new List<IOption<string>>();

// call a function that can infer the type of generic arguments from `GenericThing<>`
Type reflected = SolveGenericTypes(typeof(GenericThing<>), "solveMe", someExpression.GetType()));

// use that type to create an instance of GenericThing<string>
object instance = Activator.CreateInstance(reflected);

How could I implement SolveGenericTypes() to 'solve' for T? (Bonus, also solve for multiple generic arguments) I would need the function to work not just for this case, but basically take any generic type definition and any concrete type (assume there are no typechecking errors) and return a type that can be instantiated.

Thanks!