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I usually use almonds and canellini beans (I've found the wetter beans means it binds a bit better. Tried chickpeas a few times but it has ended up a bit crumbly). Like Ed said, you don't really taste them, they are just for the texture etc. You could maybe try oats? That's what I use to bulk out bean burgers (blitz them a bit in the food processor first), and in theory it would well work as a filling. I usually add a few tablespoons of ground flax to the mix too, which will help the binding as well.
I'm not the world's biggest fan of nuts in savoury food, or of walnuts in general. Is there anything you would recommend for a replacement?