I am a bit confused on Itialian surnames that begin with di and de with a space before the last name. Examples: Robert De Niro, Portia di Rossi, Leonardo DeCaprio (no space after De), and di Medici.
Then there's da Vinci!
Can some please explain why these prefixes are sometimes capitalized and ...

can someone please help me to pronounce the last name "Lasia."
Is it LAH-see-ah, with the stress on the first syllable? Or Lah-SEE-ah, with the stress on the middle?
Please write the name out phonetically and tell me the correct pronunciation!
Thank you for any help.