Jest - mocking imported function from another file not working

I have been trying to mock a function which is imported in another file and used in a class. There are similar questions here and I went through a lot of them but still failed to get my tests working. Here is the structure of my code:

//util.js
export const stageHelper = (key) => {
    return STAGE_NAMES[key];
};

//main.js
import {stageHelper} from './util'
class Main {
    static config = {
        value: stageHelper('abc'),
    }
    
    static getValue() {
        return this.config.value;
    }
}

//main.spec.js
import * as util from './util';
import {Configuration} from '../configuration/configuration';
jest.mock('./util');

describe('Main', () => {
    const utilSpy = jest.spyOn(util, 'stageHelper').mockImplementation(() => 'testValue');
    //util.stageHelper = jest.fn().mockImplementation(() => 'testValue'); // tried this too
    //utilSpy.mockReturnValue('ja'); // tried this too
    
    expect(Main.getValue()).toEqual('testValue'); // this test fails - the value is 'undefined'
});

I get undefined when calling Main.getValue() from my test. However, I expect this to return testValue since this is what I have mocked the return value as.

Could someone please help me get this right? This would be much appreciated!