I've only ever gotten take-out or delivery around lunch time, and Lemon Grass isn't always the most reliable of spots (they've managed to lose an order I phoned in and have sometimes delivered incomplete orders), but what they lack in consistency, they make up for in taste.
Billed as Cambodian Cuisine, their lunch menu gives a large choice of items for fans of Thai and Chinese food as well. I'll often go with their delicious green curry or satisfying Pad Thai (not the best Pad Thai Pittsburgh has, but the best downtown lunchtime offering I've found). All lunch specials come with soup (small but fulfilling) and spring roll (tiny but delicious), which makes the price (around $7) a pretty good deal.