Or actually, to take away from the pantomime villain storyline, back to a proper account of coercive control where the person assumes everything is there fault and gets blamed for everything.
Most plausible scenario is that Ursula always puts temperature on max to heat the fat quickly, then turns oven down once the toad go in. That's what I always do.
Then forgets to tell Helen she also needs to turn the oven down. In most circumstances Ursula would say 'oh no, I'm so I didn't tell you' but in a toxic relationship it's much easier to say I don't know what happened so it must be your fault, and for Helen to immediately start from that defensive position.