Parcooking carrots and green beans for an already well-seasoned casserole
In my experience, carrots at about 1.5-2cm thickness will be cooked through after an hour at a bare simmer. You might consider slightly browning, rather than boiling them before adding them to the mixture, for flavor, and to get them up to temperature. As for green beans, (depending on how you like them ) add them 10 -15 minutes before the end of your cooking time?

But if you really don't want to shift the flavor, why not cook them exactly to your liking and add them right at the end, just before service? More like a garnish than an ingredient. Plenty of traditional French stews do this with bacon, caramelized onions, mushrooms .. etc.