Let's assume that
a) I'm making a pasta sauce with salmon and creme fraiche (for two)
b) I'd like to bulk it up so there's enough left over for another meal (for two)
c) I have plenty of salmon
d) I only have a 250ml pot of creme fraiche
d) I can't be arsed going back out to the shop for more
Can I use milk to bulk it up? Or will it go a bit weird?
Surely milk would "water" it down?
You can though I've done similar using a bit of cornflour too (mixed into a bit of cold water before adding) to help avoid it being too watery/thin.
It'll be a bit less creamy tasing but IME it's perfectly acceptable.
[img] 
[/img]
What about a tin of tomatoes, so it's a tomato, creme fraiche and salmon sauce?
I don't know about the fishy stuff, but I've made white sauce for pasta primarily using milk and cornflour before now and it's worked fine.
Pint of milk, half an onion roughly chopped, couple of smashed cloves of garlic, bit of cinnamon, salt, pepper, bay leaf. Simmer for a few minutes, then leave to stand infusing for 10 mins or so. Sieve, put back on heat and stir in 3tbsps of cornflour (mixed into cold milk first), heat it through stirring often as the flour thickens up.
For carbonara-esque pasta, I'd add chopped spring onions, 'chicken' (Quorn strips), and a good handful of spinach.