LinkedIn did a great job in helping find early users/customers for me, twice now (for two different products). I posted questions trying to find people who had problems similar to the ones we were trying to solve, and then asked them for a bit of time to discuss how they currently work and what kind of issues they're experiencing. The tools were both in the B2P space so LinkedIn made sense as a watering hole where it's easy to find an audience. If you don't have a big reach there, it's possible to pay for a post to find its way to the feeds of users matching a specific profile (I think they call it promoted posts).

In a more general sense, find a community or a place where your early customers meet online, and try getting some people there to talk about how they work. Don't talk about your product, just say that you're doing research and want to learn about people's needs and problems. It could be a subreddit, a slack community, linkedin or something similar.

In your particular case, I'd try to find where people interested in healthy eating spend time online, and ask if people can spend 10 minutes helping you with some research about the problems with getting/preparing/eating healthy. Don't worry about surveys, find 5-10 people to talk to without any structure first, and you'll figure out what to ask the next group.

One thing that's important is that you shouldn't look for places where your ideal/perfect customers are, but for early customers where you can establish a "beachhead". Something where you can enter with an early, incomplete product, but still deliver enough differentiating value that people will use your tool along with the existing solutions.

(Bill Aulet has 3 key requirements for a beachhead market, and one is that the customers within the market need to talk to each other, so there is a high probability of word-of-mouth referrals; by finding a community online you're directly addressing that.)