Fresh grad started working for a Company B and gets onsite interview for dream role at Company A

Three weeks ago I had a phone interview with Company A and they did not respond for a long time and around the same time I had an onsite interview with Company B. Two weeks ago I accepted a job offer

Three weeks ago I had a phone interview with Company A and they did not respond for a long time and around the same time I had an onsite interview with Company B. Two weeks ago I accepted a job offer for Company B as their offer letter had a deadline. The joining was within the week and I stared working in Company B from this week. There was no communication from Company A all this while. Yesterday Company A invites me onsite for an elaborate interview for my dream development position, while Company B has offered me a engineering support role. There is not going to be huge difference in the pay (both are good). But I feel that continuing in Company B would not lead me to a development role anytime in the future. I took the offer from Company B as I didn't have any other option then. So my main question here is during the onsite interview if Company A asks do you have any other offers, how should I be handling it? By the time of interview, it would have been about a week since I joined Company B. Also I don't find the work at Company B challenging or enriching me in any way and matches my profile about 70% and I will get to use 50% of my skillset, while on the other hand Company A's job profile is super awesome and matches my skills 100% and I will get an opportunity to use 100% of my skillset. TL;DR: Fresh grad started working for a Company B and gets onsite interview for dream role at Company A. How do I deal with HR? What should I tell them; what should I not?