Mocking read-only properties for a class

Reading from the documentation, it wasn’t clear how I could mock a single class instance method when it’s on read-only mode. I’m using MobX for our state store so I have little control over class methods:

class ViewStore {
  @observable field = null;  

  @action doOneThing = () => {
    this.field = 'one';
  }

  @action doAnotherThing = () => {
    this.doOneThing();
  }
}

const store = new ViewStore();

If I want to write a test for store.doAnotherThing and test that store.doOneThing gets called once, I can’t do it with store.doOneThing = jest.fn() as it’s a read-only property (TypeError: Cannot assign to read only property 'doOneThing' of object '#<ViewStore>').

With Sinon’s spy I could do something like spy(store, 'doOneThing'); but I haven’t figured out how to accomplished this with Jest.

Any tips?