It seems I have found the solution with the help of this answer:

type Foo = Record<string, any>;
type FuncGenericReturn<T> = () => T;

const untypedFunction = () => {
    const test = {};
    // ...
    return test;
}
const funcReturnsFoo: FuncGenericReturn<Foo> = untypedFunction;

const myObject = funcReturnsFoo(); // myObject is typed as Foo