Can a court order that a will be 'proved in solemn form' in Ontario, and what does that involve?

Yes. When a will's validity is genuinely in dispute, an Ontario court can order that it be "proved in solemn form" — a formal court process, rather than the more routine, largely administrative way most wills are accepted for probate. Instead of simply filing the will with supporting materials, the person propounding the will (usually the proposed estate trustee) must actually establish, with evidence, that it meets the legal requirements for a valid will: proper signing and witnessing, that the testator had testamentary capacity, and that they knew and approved of its contents, free from undue influence or fraud.

This typically involves calling the witnesses who signed the will, and often the lawyer who drafted it, to give evidence about the circumstances in which it was made. It can also involve medical evidence about the testator's capacity at the relevant time, and disclosure of relevant documents from everyone involved. Proof in solemn form is essentially a trial on the will's validity rather than a paper-based application, so it is more time-consuming and costly than an uncontested probate application. Courts order it where there's a genuine, evidence-based concern about the will, not merely because someone is unhappy with the outcome.