need a substitute for goat cheese in an onion tart

Well, off hand i'd think you'd want a blend of cheeses. 

Something soft with a little tang like a Gournay (Boursin), Neufchatel or Cream cheese. This might even be a time for low or non fat Neufchatel/Cream cheese. They have more sourness and more goat cheese like texture.

Something with some salt. Feta was a good choice as it brings some crumbliness to the texture. Pecorino would be useful here too probably to get some of those sheepy/goaty tones (but maybe not if they don't like Pecorino either) Ricotta Salata would be OK too.

Then some medium white cheddar or maybe a jack cheese to give some body and texture to the mix without too much distinct flavor.

Phoebe, for me, I love goat cheese, but my husband does not. 

He doesn’t like the texture or the tang, as he says, “it’s sour, I don’t like it”. 

For that matter, he doesn’t like anything that’s sour. 

So I use a lot Parm, naturally Mozz on the pizza and ricata in the lasagna, oh and Swiss for that Ham & Cheese!

I've done a similar tart with sour cream, some egg to help it set up. You could get away with cream cheese just fine, sort of a savory cheesecake. I think I'd still add a little pecorino just because I like it.

Make yogurt cheese. It will have a tang and not be too salty. Buy a yogurt free of stabilizers or gelatin (chobani is an example). Line a colander with several layers of cheesecloth and place into a larger bowl on top of an inverted bowl so that drainage can occur. Pour the yogurt into the colander. Cover with cellophane and refrigerate overnight.

You can try mixing ricotta cheese with a little feta cheese, that way the salt is very mellowed out yet you still have the tartness of the feta.  Maybe mix some cream cheese in it as well?  I think that would pair well with the sweetness of the onion and the salty smokiness of the bacon.