Great episode, as usual.

But if I may...
Foie Gras is not grey, it's beige. Plus, it's not a paste, more like, hum... Imagine you froze multiples lumps of butter, and then press them together, and then let it chill on your kitchen's table for just enough time to make the pieces stick together, but the pieces itself still soft. Yeah, hard to describe, but that's close enough.
Anyway, it's not a mouse, and it's not grey.
And most of the time, it's served with sugary sauce like Onion Jam or Figs Jam (and a dash of sea salt, but I disgress).
BUT
what you said about it is, indeed true (well, except for gavage. I don't say it's nice to animals, but it's horrendous when done at an industrial level. Like the KFC chikens or any industrial level food. But go figure why politicians don't ban industrial food...)

So what is it?
Well, I vote for Mushroom Mouse.
It have the problem to be cheap. Other than that, it fits the bill perfectly.

But hey, that's just a theory, A COMMENT THEORY!
Bon Appétit! (Bone App Hay Tea)

Bonus, here is a recipe (in French):
https://www.cuisineaz.com/recettes/mousse-aux-champignons-74927.aspx
Here is my take on the recipe:
If you aim for a grey color, keep the black part of the mushrooms.
Sauté a minced shallot (you can use onion/red onion as an alternative), and a brunch of Paris mushroom (portobello as an alternative) in butter (please, don't use olive oil. You can use lard fat, it's even better. And if you really carve for a vegan alternative, go for a neutral taste oil, like sunflower, or maybe denatured coconut oil?).
When it's all soft an cooked, mix everything (you can add a dash of milk/whole cream. Not much, it's just to soften a bit, and give some creaminess. For vegan, just add nothing. Please, no almond or sugary soy contraption. If you can find a good substitute, go for it, but you should have tasted the original beforehand to have a comparison. I guess that you can try your luck with a dash of fresh cashew "fauxmage" cheese? It could fit the bill, but I never tested it).
Wipe some egg white to foam it and incorporate you mushroom preparation (for vegan, well, I guess you can try chickpea foam. Maybe add a dash of agar agar to have a better consistency).
I find it better to chill it a bit to help gain some consistency.
You can now spread it on garlic scratched grilled bread / toasts.
And of course, you can add a dash of parsley and/or sea salt as a topping.
Enjoy your starter!