If I had to pick out anything absolutely essential, stand alone compnent to a North Indian meal, I'd point to the roti (chapati) or the unleavened tortilla-like flatbread. This may sometimes be the only ingredient in a thali; and yet enough for sustenance.
And I'm not talking only about those who can't afford to pay for food. Growing up, I had a cousin who'd only eat his roti with ghee-boora (powdered sugar mixed with butter). My treat at my naani's home used to be roti rolled around a thick spread of fresh malai (milk cream) sprinkled with sugar. My mom loved to coat her rotis with ghee- mirch (red pepper powder mixed with clarified butter) with a raw onion on the side. And on hot summer days, I remember coming home from school and sitting down for a lunch of Rajasthani Missi Roti with a glass of chilled Matha (spiced butermilk). This roti was the informal, no-frills attached at-home thali.
This missi roti was something that my mom would never offer her guests; because of the 'special' status that described 'athithi' in her mind. Looking back, I don't think I've had this roti for over a decade now. Which is probably why I lost the reference point for this meal from my mind in the first place. Then recently, at one of our Swadhyay picnics, we were with friends and somehow the conversation turned to 'what reminds us of our family time from our childhood?' I can safely assume that more than half the inputs were food related. When some one said 'khichdi', because it gave their mom a chance to sit and eat with the family, it instantly flashed a picture of my family sitting together over missi roti. It was such a strong association, that I just fell compelled to make my mom's Rajasthani Missi Roti for dinner one night.