That depends on whether I'm speaking English or Dutch.
If it's English, I say AS-per-gur. The 'g' pronounced as in 'goal'. 'As', like the word 'as'. 'Per', like the word 'per'. Stress on the first syllable.
Before it comes up, I hardly ever employ the German pronunciation of the name. Maybe if I ever have to talk about it in German.
Edit: I pronounce the 's' unvoiced (i.e. as an 's', not as a 'z'). So I really pronounce the first syllable as 'ass'. I want to make an immature joke, but dignity prevents me.
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