Can my insurer settle a claim against me without my consent in Ontario?

Usually, yes, within limits set by your specific policy. When an insurer accepts a claim without reservation and takes over your defence, most liability policies give the insurer the right to control the defence, including deciding whether and on what terms to settle, since it's the insurer's money being spent. Some policies include a consent clause requiring your agreement before a settlement, but where the policy gives the insurer control, your ability to block a reasonable settlement within policy limits is generally limited.

That control isn't unconditional. If an insurer settles unreasonably against your genuine interests, or refuses a reasonable settlement offer within policy limits and you end up facing a larger judgment as a result, that can potentially support a claim against the insurer for the resulting exposure. Whether a settlement decision was reasonable is judged against what a prudent insurer would have done in the circumstances, not simply whether you personally would have preferred a different outcome. Reading your specific policy's wording on settlement authority and consent is the necessary first step if you're concerned about how your insurer is handling a settlement decision.