Gaining weight in a healthy way?

I’m looking for some diet advice.

I currently cook dinner every night from scratch (pasta dishes, enchiladas, tacos, arepas, stir fry, soups, etc). I use all organic foods, sustainable meat and seafood, and high quality ingredients because cooking is my passion. While my husband is at work and I’m home with my daughter, I usually drink coffee throughout the day and eat leftovers for lunch, or sandwiches. However, I cannot put on weight to save my life. I thought after having a baby, I’d get some curves. NOPE.

I don’t want to stop eating clean, I don’t want to stop exercising, and I’m not a sweet eater... is there a secret to keeping your weight up that satisfies these things?

I’ll also add that I do eat a healthy quantity at dinner, and I also have tried to use more red meat.